Support blank EngineAssociation

[brasky]

Blank EngineAssociation should be handled as follows:

For Perforce or Git users that branch the engine along with their games, this field is left blank. You can sync the repository down on any platform and machine, and it can figure out which engine a project should use by looking up the directory hierarchy until it finds one.

https://docs.unrealengine.com/4.26/en-US/API/Runtime/Projects/FProjectDescriptor/EngineAssociation/