This PR fixes a bug came from assuming the old null check only mattered
for the `is_not_present` filter.
The fix keeps `not_equal_to` working but lets each operator decide what
to do with `null`. Presence filters look at a shared `isNullish` flag,
text filters still rely on `contains`, and date filters skip
conversations with no timestamp. The new spec covers the null-assignee
scenario for both `equal_to` and `not_equal_to` so we don’t miss this
again.
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