Handle incomplete protectors in destroyAllWeakTokenBasedProtectors()

A partner reported a NullPointerException in
SyntheticPasswordManager#destroyAllWeakTokenBasedProtectors() because
loadState(SP_BLOB_NAME, ...) returned null.  I think the only way this
could happen is if an incomplete protector exists on-disk due to a
crash, containing the secdiscardable file but not the spblob file.

It does seem theoretically possible, though, so make
destroyAllWeakTokenBasedProtectors() handle this case by filtering the
protectors by the existence of the spblob file.

Bug: 285241930
Change-Id: Ia148234aa332ca716dcb208a814ae2d3b3c9f6b7