[IndexedDB] Check if rfh is null after retrieving from rfh_id

In crrev.com/c/3980833, we implemented `IndexedDBClientChecker`, which
requires the RFH id during initialization. However, after obtaining the
RFH pointer from the rfh_id, there is no nullness check which causes
potential null pointer access after the RFH is released.

In this fix, if the rfh is null, a
`NoDocumentIndexedDBClientStateChecker` will be returned instead.

Bug: 1402081
Change-Id: Iff44dfbe3977117e5827097041cdc1cc615f2784
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4113190
Commit-Queue: Mingyu Lei <leimy@chromium.org>
Reviewed-by: Fergal Daly <fergal@chromium.org>
Reviewed-by: Jonathan Ross <jonross@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1085487}