[clang] [ASTWriter] Do not write ObjCCategories if empty. (PR #141841)

[Jan Svoboda via cfe-commits]

jansvoboda11 wrote:

This change makes sense to me. I have two questions:
1. Can we add a test that checks that the block is omitted? (maybe via `llvm-bcanalyzer -dump`)
2. Can we make the `explicit-build.cpp` test more robust? (possibly by writing out import signatures)

https://github.com/llvm/llvm-project/pull/141841