Rig sqlite3_serialize() so that it will initialize a previously uninitialized

Rig sqlite3_serialize() so that it will initialize a previously uninitialized
database prior to serializing it, so that it does not have a zero-byte size
and does not return NULL (except for OOM).
[forum:/forumpost/498777780e16880a|Forum thread 498777780e16880a].

FossilOrigin-Name: e638d5e408ea2e189b6771d16bbc2e42c606e88e05fbea78079b6e39e41f344c