Fixes conversion failure when using erb template

Internally eruby option was set as null which caused Asciidoctor to fail
with 'Unknown ERB implementation'.
The fix sets the option only when a non-empty value is present, delegating
to AsciidoctorJ default which is 'erb' (consistent with current docs).

CHANGELOG.adoc

@@ -13,8 +13,12 @@ For a detailed view of what has changed, refer to the {uri-repo}/commits/main[co
 
 == Unreleased
 
+Bug Fixes::
+
+  * Fixed default value for eruby which caused a fail when using erb templates (#610)
+
 Build / Infrastructure::
-  
+
   * Bump Doxia to v1.11.1 and maven-site-plugin in IT to 3.12.0 (#579)
   * Bump netty-codec-http to v4.1.77.Final (fix CVE-2021-21290) (#582)
   * Upgrade Asciidoctorj to v2.5.4 and jRuby to v9.3.4.0 (#584)

src/main/java/org/asciidoctor/maven/AsciidoctorMojo.java

@@ -88,13 +88,13 @@ public class AsciidoctorMojo extends AbstractMojo {
     protected String attributesChain = "";
 
     @Parameter(property = AsciidoctorMaven.PREFIX + Options.BACKEND, defaultValue = "html5")
-    protected String backend = "";
+    protected String backend = "html5";
 
     @Parameter(property = AsciidoctorMaven.PREFIX + Options.DOCTYPE)
     protected String doctype;
 
     @Parameter(property = AsciidoctorMaven.PREFIX + Options.ERUBY)
-    protected String eruby = "";
+    protected String eruby;
 
     @Parameter(property = AsciidoctorMaven.PREFIX + "headerFooter")
     protected boolean headerFooter = true;
@@ -123,14 +123,14 @@ public class AsciidoctorMojo extends AbstractMojo {
     @Parameter
     protected List<ExtensionConfiguration> extensions = new ArrayList<>();
 
-    @Parameter(property = AsciidoctorMaven.PREFIX + "embedAssets")
+    @Parameter(property = AsciidoctorMaven.PREFIX + "embedAssets", defaultValue = "false")
     protected boolean embedAssets = false;
 
     // List of resources to copy to the output directory (e.g., images, css). By default everything is copied
     @Parameter
     protected List<Resource> resources;
 
-    @Parameter(property = AsciidoctorMaven.PREFIX + "verbose")
+    @Parameter(property = AsciidoctorMaven.PREFIX + "verbose", defaultValue = "false")
     protected boolean enableVerbose = false;
 
     @Parameter
@@ -476,9 +476,11 @@ protected OptionsBuilder createOptionsBuilder(AsciidoctorMojo configuration, Att
                 .backend(configuration.getBackend())
                 .safe(SafeMode.UNSAFE)
                 .headerFooter(configuration.isHeaderFooter())
-                .eruby(configuration.getEruby())
                 .mkDirs(true);
 
+        if (!isBlank(configuration.getEruby()))
+            optionsBuilder.eruby(configuration.getEruby());
+
         if (configuration.isSourcemap())
             optionsBuilder.option(Options.SOURCEMAP, true);
 

src/test/java/org/asciidoctor/maven/AsciidoctorMojoTest.java

@@ -179,9 +179,9 @@ public void should_convert_to_html_with_attributes() throws MojoFailureException
     }
 
     @Test
-    public void should_convert_to_html_with_a_custom_template() throws MojoFailureException, MojoExecutionException {
+    public void should_convert_to_html_with_a_custom_slim_template() throws MojoFailureException, MojoExecutionException {
         // given
-        final String templatesPath = "target/test-classes/templates/";
+        final String templatesPath = "target/test-classes/templates/slim/";
         File srcDir = new File(DEFAULT_SOURCE_DIRECTORY);
         File outputDir = newOutputTestDirectory();
 
@@ -205,6 +205,31 @@ public void should_convert_to_html_with_a_custom_template() throws MojoFailureEx
                 .contains("custom-block-style");
     }
 
+    @Test
+    public void should_convert_to_html_with_a_custom_erb_template() throws MojoFailureException, MojoExecutionException {
+        // given
+        final String templatesPath = "target/test-classes/templates/";
+        File srcDir = new File(DEFAULT_SOURCE_DIRECTORY);
+        File outputDir = newOutputTestDirectory();
+
+        // when
+        AsciidoctorMojo mojo = mockAsciidoctorMojo();
+        mojo.backend = "html";
+        mojo.sourceDirectory = srcDir;
+        mojo.sourceDocumentName = "sample.asciidoc";
+        mojo.resources = excludeAll();
+        mojo.outputDirectory = outputDir;
+        mojo.templateDirs = Arrays.asList(
+                new File(templatesPath, "erb")
+        );
+        mojo.execute();
+
+        // then
+        assertThat(outputDir, "sample.html")
+                .isNotEmpty()
+                .contains("custom-style");
+    }
+
     @Test
     public void should_set_output_file() throws MojoFailureException, MojoExecutionException {
         // given

src/test/resources/templates/erb/section.html.erb

@@ -0,0 +1,28 @@
+<%#encoding:UTF-8%><%
+slevel = @level.zero? && @special ? 1 : @level
+anchor = link_start = link_end = nil
+if @id
+  if @document.attr? :sectanchors
+    anchor = %(<a class="anchor" href="##{@id}"></a>)
+  elsif @document.attr? :sectlinks
+    link_start = %(<a class="link" href="##{@id}">)
+    link_end = '</a>'
+  end
+end
+if slevel.zero?
+  %><h1<%= @id && %( id="#{@id}") %> class="sect0"><%= %(#{anchor}#{link_start}#{title}#{link_end}) %></h1>
+<%= content %><%
+else
+  snum = @numbered && @caption.nil? && slevel <= (@document.attr 'sectnumlevels', 3).to_i ? %(#{sectnum} ) : nil
+  hlevel = slevel + 1
+  %><div class="<%= [%(sect#{slevel}),role].compact * ' ' %>">
+<h<%= hlevel %><%= @id && %( id="#{@id}") %>><%= %(#{anchor}#{link_start}#{snum}#{captioned_title}#{link_end}) %></h<%= hlevel %>><%
+  if slevel == 1 %>
+<div class="sectionbody custom-style">
+<%= content %>
+</div><%
+  else %>
+<%= content %><%
+  end %>
+</div><%
+end %>