Post commit fixes

This CL introduces a hotfix post refactoring of NestedMatchers:
- fix uninitialized read to skip
- avoid bumpptr allocating with 0 elements

Interestingly the latter issue only surfaced in fastbuild mode with no-san and
manifested itself by a SIGILL. All other combinations that were tried failed to
reproduce the issue (dbg, opt, fastbuild with asan)

PiperOrigin-RevId: 231787642