AO3-6796 - fix QA returns for 500 error on fandom_id

[AliceLoeser]

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Issue

https://otwarchive.atlassian.net/browse/AO3-6796

Purpose

This PR is a follow up of this previously merged PR, after QA analysis: #4898

The scenario this PR fixes is the following:

Tried one more test, where I completely deleted all numbers from the fandom_id (https://test.archiveofourown.org/users/testy/works?fandom_id=), which resulted in an Error 500!

Now, fandom_id will be ignored if null.

Testing Instructions

Look at the test scenarios described in this issue: https://otwarchive.atlassian.net/browse/AO3-6796
And test again for "Tried one more test, where I completely deleted all numbers from the fandom_id". Expected behavior is "the behaviour should be to show the /works page as if the fandom_id wasn’t specified."

References

This PR is a follow-up of: #4898

Credit

AliceLsr (feminine pronouns)

[brianjaustin]

Thanks!