Update utils.py

[hho6643]

verify get_meta isn't none before requesting db_table

Description

Patch to check that "query.get_meta()" is not None before getting db_table attribue.

Rationale

Using:
Django: 4.1.6
django-cachalot: 2.5.2
psycopg2: 2.9.5

During Model.validate_constraints() on new object save, I'm getting an "AttributeError" exception in _get_tables of utils.py at:
tables.add(query.get_meta().db_table); specifically when checking a FloatField constraint is greater than 0.

The issue is query.get_meta() is returning None, which doesn't have a db_table attribute. This patch checks for None before getting db_table.

The issue does not occur on Django 4.0.9.

[Andrew-Chen-Wang]

Can you add a test and fix CI?

[hho6643]

Can you add a test and fix CI?
Hi - Yes, I can add a test.

If I'm reading the CI issue correctly, though, it looks like errors keep getting generated because of a missing password for the DB. That seems like a separate problem unrelated to this patch? It's very possible I'm not reading this correctly.

psycopg2.OperationalError: fe_sendauth: no password supplied

[Andrew-Chen-Wang]

@hho6643 I've fixed the CI errors. Would be great to get a test written here if your time permits. Thanks again!

[coveralls]

Pull Request Test Coverage Report for Build 4383434952

• 2 of 2 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.004%) to 97.202%

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Totals
Change from base Build 2937621086:	0.004%
Covered Lines:	660
Relevant Lines:	679

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💛 - Coveralls

[PetrDlouhy]

@hho6643 I have written test that might be relvant here in #227, although there is one issue left to solve there.

[hho6643]

Hah, I finally got around today to start working on a test and it's already done!