Fix source query caching

[obulat]

Fixes

Follow up fix of change in #3300

Description

@dhruvkb has flagged that the condition for the sources stats aggregation ES query caching is incorrect and is always True. This means that we always query ES instead of using the cached value.
This PR removes the incorrect or sources to use cache when the sources list are empty in cache.

Testing Instructions

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[sarayourfriend]

LGTM as is, but this looks like a good candidate for a unit test, considering we missed it in review before 🙂

[dhruvkb]

LGTM, but reiterating @sarayourfriend's request for a test. Not as highly required (because #3505 adds some) but good to have.

Additionally, this line https://github.com/WordPress/openverse/pull/3516/files#diff-8429482fb10e7f976ad2295c754632b960e3edc0dae9edfa29504cbabf24d967L598 yields a weird response where I believe a blank array would be more appropriate.

- buckets = [{"key": "none_found", "doc_count": 0}]
+ buckets = []

[obulat]

Additionally, this line https://github.com/WordPress/openverse/pull/3516/files#diff-8429482fb10e7f976ad2295c754632b960e3edc0dae9edfa29504cbabf24d967L598 yields a weird response where I believe a blank array would be more appropriate.

If we use a blank array, the next line in that file would raise an error.

[dhruvkb]

If we use a blank array, the next line in that file would raise an error.

It doesn't.

Nevermind, it's merged now.