[clang][objc][codegen] Skip emitting ObjC category metadata when the

category is empty

Currently, if we create a category in ObjC that is empty, we still emit
runtime metadata for that category. This is a scenario that could
commonly be run into when using __attribute__((objc_direct_members)),
which elides the need for much of the category metadata. This is
slightly wasteful and can be easily skipped by checking the category
metadata contents during CodeGen.

rdar://66177182

Differential Revision: https://reviews.llvm.org/D113455