Scope Omit the "CXX=" from the handshake if "MONGOC_CXX_COMPILER_ID" is an empty string (to signal no check was made)

Background & Motivation The CXX string was added in CDRIVER-6091. Quoting https://github.com/mongodb/mongo-c-driver/pull/2114:

If a C++ compiler is not found, CXX=Unknown is included. Alternatively: CXX could be omitted if not detected. But I expect it may help with analysis to easily distinguish between: "driver too old to check" and "checked and did not find".

PHP does not check for a C++ compiler. Suggested in this comment to use an empty string to indicate "did not check". Since including "CXX=" is not really helpful (and adds data to the limited size handshake), this ticket proposes excluding "CXX=" when empty.