Protobuf CMakefiles don't work when protobuf libraries aren't installed

Take the android_content_uri test in qtbase/tests/manual/android_content_uri/
, try to configure and build it.
If QtGrpc is installed, this fails if protobuf libraries are installed, CMake tries to use the cmakefiles of Qt protobuf support, failing in protobuf tools. That then leads to the configure of such a test failing, because it's trying to use Qt6BuildInternals.

Removing Qt Grpc and nuking the build of the test and reconfiguring it works.

This looks like the detection of protobuf libraries isn't correct, and if they're not detected, protobuf support isn't disabled.