automoc: ranlib warning with static libraries

when automoc-ing a static library results in an empty mocs_compilation.cpp, ranlib on macos warns about an empty object file:

/Applications/Xcode13.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/ranlib:
file: path/To/Library.a(mocs_compilation.cpp.o) has no symbols

possible fix: !7296 (merged)