Fix crash on FieldValueComparison if not saved properly

It is possible to end up with NULL in the database instead of the condition parameters (if you start to create a new condition and then change to another via the UI this can happen).

This PR cleans up the comments/docblocks in the first commit. In the second commit it fixes the crash so you can edit and save the condition - otherwise the page just crashes and you can't do anything.

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