ASTConverterTest(s): do not use not existing jclMin1.8.jar

prevents logged java.nio.file.NoSuchFileException:
eclipse-testing\test-eclipse\eclipse\jdt_dom_folder\jclMin1.8.jar

org.eclipse.jdt.core.tests.model/src/org/eclipse/jdt/core/tests/dom/ConverterTestSetup.java

@@ -145,6 +145,9 @@ public void tearDownSuite() throws Exception {
 		super.tearDownSuite();
 	}
 
+	protected String getExternalJCLPathString(String compliance) {
+		return null;
+	}
 	@Override
 	public void setUpJCLClasspathVariables(String compliance, boolean useFullJCL) throws JavaModelException, IOException {
 		if (useFullJCL) {

org.eclipse.jdt.core.tests.model/src/org/eclipse/jdt/core/tests/model/AbstractJavaModelTests.java

@@ -2535,7 +2535,11 @@ public void ensureChildExists(IParent container, IJavaElement child) throws Java
 	protected String[] getJCLLibrary(String compliance) throws JavaModelException, IOException {
 		// ensure that the requested JCL lib is setup (i.e. that the jclMinXY.jar is copied)
 		setUpJCLClasspathVariables(compliance);
-		return new String[] {getExternalJCLPathString(compliance)};
+		String externalJCLPathString = getExternalJCLPathString(compliance);
+		if (externalJCLPathString == null) {
+			return new String[] {};
+		}
+		return new String[] {externalJCLPathString};
 	}
 	/**
 	 * Returns the specified compilation unit in the given project, root, and