PostgreSQL: decode image.img_metadata

The img_metadata field was not being decoded from bytea into text.

This change to core fixes the reported problem with PdfHandler against
1.22.6, origin/REL1_23 and git head.

Bug: 59147
Change-Id: I285a317a57a29713af1f7f630b340278bd5400da
(cherry picked from commit 8c62609dc877b6ed687e9a3835cfc24fb87e5be2)
Merge "PostgreSQL: decode image.img_metadata" into REL1_22