When doing the optimization that attempts to avoid sorting on a

When doing the optimization that attempts to avoid sorting on a
GROUP BY, do not assume that the values in an index on an expression are
non-NULL.  Bug discovered by Wang Ke's fuzzer and reported at
[forum:/forumpost/74330094d8|forum post 74330094d8].

FossilOrigin-Name: 7178dc3a32c3a4a33e437fd5026f6f72e4809ceb7d2cd12a25a74b80d5c95d33