Minor bug fix for tests.WarningChecker when IERS is down

[kartographer]

Fixes testing bug.

Description

I've updated the WarningChecker class inside of pyuvdata.tests to handle when the IERS service is down and when check_warnings is being called with the expected warning and text match being set to None (which effectively ensures that no warnings are being thrown).

Motivation and Context

There are about half a dozen tests which use check_warnings to make sure no warnings are being thrown, which were failing due to the previously uncaught behavior. This sems to be a fairly rare occurence that's popped up recently due to what appear to be IERS server issues.

Types of changes

• Bug fix (non-breaking change which fixes an issue)

Checklist:

• I have read the contribution guide.
• My code follows the code style of this project.

Bug fix checklist:

• My fix includes a new test that breaks as a result of the bug (if possible).
• All new and existing tests pass.
• I have updated the CHANGELOG.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (fb99885) 99.92% compared to head (c67a546) 99.92%.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1382   +/-   ##
=======================================
  Coverage   99.92%   99.92%           
=======================================
  Files          37       37           
  Lines       20423    20423           
=======================================
  Hits        20407    20407           
  Misses         16       16           

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[kartographer]

Note that this seems to be a different error from the one impacting the warnings ci. This problem was causing intermittent failures on the other pipelines.

[bhazelton]

Looks reasonable. Thanks!