Only update DebugMutator and DebugVariation for non-empty transition variations

Non-transition mutators that don't create a variant for a given module
cause no change to DebugMutator and DebugVariation.  TransitionMutators
don't have a concept of skipping a module, so Mutate is called on
every module.  This causes extra entries in DebugMutator and DebugVariation,
which will confuse error message around missing variations, and require
updating variant strings in some tests.

Skip updating DebugMutator and DebugVariation for empty transition
mutator variations.  This isn't entirely correct, but transition mutators
don't currently have a concept of an unaffected module.

Bug: 319288033
Test: all soong tests pass
Flag: NONE
Change-Id: Ia4e659b1e90ffe4320074c9b49528d00fa302c6a