Check pgSQL image tag exists before checking version

General MR

Summary

One of the helm checks is failing in the release cluster because .Values.postgresql.image.tag was commented out in the latest update and the semver func doesn't know how to handle null input. Added an check to confirm the field is set before running the test.

Relevant logs/screenshots

Linked Issue

big-bang/bigbang#2738 (closed)

Upgrade Notices

N/A