Don't crash if type is not available in Python Doxygen code

Fix crash if "@return" Doxygen tag was used on a node without any return
type (such as a class, for example). Ignoring it might not be the best
thing to do, but it's definitely better than crashing and it's not
really clear what else could be done anyhow.

Closes #1516.

Examples/test-suite/doxygen_misc_constructs.i

@@ -104,6 +104,9 @@
 
     };
 
+    /// @return This is a bad place for this tag, but it should be ignored.
+    struct StructWithReturnComment {};
+
     /**
         An example of a list in a documentation comment.