FIX: Ensure full absolute tagsfile filespec in async mode.

On Windows, tagfiles() can return a filespec that is absolute to the current drive (i.e. \foo\bar\tags). In async mode, the forked Vim process may have another current drive, so it should be ensured that the filespec is a full one, including the drive letter: D:\foo\bar\tags.
FIX: Avoid error about empty Dictionary key.

I had a degenerate tags file that had *two* tabs separating the tag from the filespec column; parsing that yields an empty filespec, which caused E713. There used to be an explicit check for that; with your recent refactorings, one instance of that check was lost. Here it is again!
Merge pull request #86: Ensure full absolute tagsfile filespec in async mode
Merge pull request #87: Avoid error about empty Dictionary key
Bug fix for invalid tags filtering (cache.exists() was broken, now fixed)