mi: Add a default no-op miSourceValidate

Slightly simplifies the callers since they don't need to check for
non-NULL anymore.

I do extremely hate the workarounds here to suppress misprite taking the
cursor down though. Surely there's a better way.

[1.20: Do not in fact simplify the callers as above, since it would
change the ABI. - ajax]

Reviewed-by: Michel Dänzer <mdaenzer@redhat.com>
(cherry picked from commit ff310903)