Preserve Anonymous authorship if uid is zero rather than unset.

fieldtool_execute_submit() was overwriting anonymous authorship with global $user if $node->uid evaluated to FALSE in a boolean context. I found the fact that this meant we couldn't save nodes as owned by uid 0 problematic in at least one situation, and after wracking my brains I couldn't think of a case where the inability to do this would be desirable. Have committed this to dev; now it will only use global $user if node->uid is unset, but I won't mark this issue as fixed until the next (overdue) release, in case anybody is depending on this quirk. Speak up now!