hg: make "_outgoing()" return empty list instead of "None"
This patch makes "_outgoing()" return empty list instead of "None", if
there are no outgoing changesets, because:
- returning "None" requires callers to examine whether returned
value is "None" or not explicitly, if callers want to execute loop
on returned value, but
- there are no explicit needs to return "None"
Testing that hghave does not crash when checking features
$ "$TESTDIR/hghave" --test-features 2>/dev/null