Don't display version or revision in single-file plugin if keyword is not expanded

[Ryan J Ollos]

Single-file plugins use the version and revision keywords to specify the plugin version, and this version is displayed on the plugin admin page. The keyword may be set to an svn keyword that gets expanded, however the svn keyword won't get expanded on t-h.o if the source is downloaded in a zip, or the SVN repository is cloned to Git. This leads to only Rev being displayed. In this case, it would be better to display nothing.