links: Support null "title" attribute with --reference-links.

html2text/__init__.py

@@ -823,8 +823,7 @@ def o(
                             + "]: "
                             + urlparse.urljoin(self.baseurl, link.attrs["href"])
                         )
-                        if "title" in link.attrs:
-                            assert link.attrs["title"] is not None
+                        if "title" in link.attrs and link.attrs["title"] is not None:
                             self.out(" (" + link.attrs["title"] + ")")
                         self.out("\n")
                     else:

test/no_inline_links_example.html

@@ -7,3 +7,4 @@
 <a href="http://example.com" title="abc">
 link text
 </a></a></a>
+<a href='http://example.com' title>Empty link title</a>

test/no_inline_links_example.md

@@ -1,8 +1,10 @@
 [Googler][1] No href No href but title available [ Example][2] [ [ [ link text
-][3]][3]][3]
+][3]][3]][3] [Empty link title][4]
 
    [1]: http://google.com
 
    [2]: http://example.com (Example title)
 
    [3]: http://example.com (abc)
+
+   [4]: http://example.com