run-tests.sh: don't call go test if there are no tests to run
run-tests.sh: don't call go test if there are no tests to run

Addresses the case where the modified files are not part of
any Go package.

Change-Id: I7a4289355c4d9b6d13adb5028059e4d0198cfe89
Reviewed-on: https://code-review.googlesource.com/16530
Reviewed-by: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>
Reviewed-by: Sarah Adams <2b342d1c32491b4e7587e4a0e291a5f0f95d4167@google.com>

googleapis/google-cloud-go,googleapis/google-cloud-go,googleapis/google-cloud-go,googleapis/google-cloud-go