`FeatureLibrary`: Only create empty GOL if a tileset URL is specified

[tordanik]

I've had a .gol file created when I attempted to read a non-existing file. This is undocumented for this particular constructor, and at least for me it was surprising behavior.

[clarisma]

Good point, thanks!

Unless the caller supplies a tileset URL from which tiles can be fetched, creating an empty GOL makes no sense.

• Depends on Store: Support more options for open() #89