Show license text in EditorOverlay only, if there is a license

Not all wikis (e.g. thirdparty wikis) have set wgRightsText to license
the texts. Show license message in EditorOverlay only, if there is a license
to avoid confusing text.

Change-Id: I86e6dbfbc3097a086b38b5519f0fbf0c32e9cd41
Merge "Show license text in EditorOverlay only, if there is a license"