transport/grpc: Fix check of GOOGLE_CLOUD_ENABLE_DIRECT_PATH
transport/grpc: Fix check of GOOGLE_CLOUD_ENABLE_DIRECT_PATH

When GOOGLE_CLOUD_ENABLE_DIRECT_PATH is empty and split is called
a slice with a single empty string is then looped and matched
with strings.Contains. This means that without that environment
variable set every single endpoint is matched.

Instead check to make sure the match isn't against an empty string
first.

Change-Id: I7382c7997098a9f77db44bc8e1fb10942caeaa61
Reviewed-on: https://code-review.googlesource.com/c/google-api-go-client/+/41670
Reviewed-by: Jean de Klerk <605b93664484fed1e671a07bff68da39e73d0174@google.com>

google/google-api-go-client,google/google-api-go-client,googleapis/google-api-go-client,googleapis/google-api-go-client