swim: fix crash on bad member_by_uuid() call

In Lua swim object's method member_by_uuid() could crash if called
with no arguments. UUID was then passed as NULL, and dereferenced.

The patch makes member_by_uuid() treat NULL like nil UUID and
return NULL (member not found). The reason is that
swim_member_by_uuid() can't fail. It can only return a member or
not. It never sets a diag error.

Closes #5951