Query: correctly marshal errors to JSON and ignore if nil

[cuotos]

• I added CHANGELOG entry for this change.
• Change is not relevant to the end user.

Changes

Fixes: #2846
Passing nil error to updateStoreStatus causes a panic. This is displayed to the user in the Stores ui when the Store is healthy

Added higher level unit test to catch this, and hopefully clarified existing test.

[cuotos]

thought id completely message up my testing, panicked and closed this. turns out i had another instance running.... have created a new PR