postgresql:  Check if value is a string before checking if it starts with $
postgresql:  Check if value is a string before checking if it starts with $

Missed this in the previous commit as I was testing with env var emulation
NewRelic-Python-Plugins/newrelic-python-agent,NewRelic-Python-Plugins/newrelic-python-agent,NewRelic-Python-Plugins/newrelic-python-agent