fix: detect correct auth when ADC env var is set but empty

[mbrancato]

Treat empty strings as if the env var was not set. This fixes an issue where an environment variable for GOOGLE_APPLICATION_CREDENTIALS is set, but it empty. Because the default sentinel is None, the auth library attempts to load this file. Some systems can set or update an env var, but not unset an env var (e.g. Docker).

Fixes: googleapis/python-pubsub#983

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