* Fix ec2 filter by empty tag value
Return `None` instead of an empty string when the tag key does not exist
and replace the falsy check with a more explicit `is None`, which allows
empty string values to correctly pass through the filter comparator.
Behavior confirmed against a real AWS backend.
Closes#3603
* Make test case more explicit
Test case now pulled directly from the issue report (#3603).
Co-authored-by: Bert Blommers <bblommers@users.noreply.github.com>
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