Prefer empty() over size()
Merge pull request #2975 from Fice/empty

Prefer empty() over size()
Include guilib/GraphicContext.h and don't assign NULL to int

Fixes: #14501
Merge pull request #2977 from smspillaz/fix-14501

Include guilib/GraphicContext.h and don't assign NULL to int
[musicdb] fix album & artist lt operators, used for set/map (e.g. cache)
[musicdb] fix some problems with non-musicbrainz albums in scanner, and tidy up some of the scanner overall.
[musicdb] fix album and artist SQL queries for musicbrainz