Fix `-Wgnu-zero-variadic-macro-arguments` in GMock

Passing zero arguments to the variadic part of a macro is a GNU
extension and triggers warnings when build projects using GMock with
`-pedantic`.

 - Fix uses of `GMOCK_PP_INTERNAL_16TH` to always receive at least 17
arguments. (this was triggered when `GMOCK_PP_NARG` or `GMOCK_PP_HAS_COMMA`
were used with an argument containing no commas).
 - Fix `GMOCK_PP_HEAD` to append a dummy unused argument so that
`GMOCK_PP_INTERNAL_HEAD` always has two arguments.

PiperOrigin-RevId: 310414611