(Bug 60753) Accept empty links in image caption.

Newly added "Allow empty links in image captions" test is failing wt2wt, 
html2wt and html2html modes because the serializer is adding nowikis in
the image caption around [[]].

Change-Id: I484b591fc0e2711d81f6f4a834889df1ca537991
Merge "(Bug 60753) Accept empty links in image caption."