FindGTest: Create the gmock targets only when GTest has been found

When we introduced the `GTest::gmock` and `GTest::gmock_main` targets in
commit 50bf457a0d (FindGTest: Add target for gmock library, 2021-10-17,
v3.23.0-rc1~321^2) we failed to handle the case where GTest isn't found.
Don't construct gmock targets that depend on non-existent gtest targets
when gtest failed to be found.
1 file changed