qt_add_protobuf and qt_add_grpc should print a warning if .proto files miss respective definitions

qt_add_protobuf and qt_add_grpc are designed to work with .proto files which contains the respective definitions of either protobuf messages or gRPC services. Currently they fail with really annoying errors in case if the only .proto file that is provided to the qt_add_protobuf/qt_add_grpc call lacks the required definitions:

Qt/6.8.1/gcc_64/lib/cmake/Qt6ProtobufTools/Qt6ProtobufToolsMacros.cmake:76 (message):
  PROTO_FILES list is empty.

We should prevent this situation and print the adequate warning regarding the missing definitions in .proto files and allow to bypass this.