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"

mercurial/commands.py

@@ -4354,7 +4354,7 b' def outgoing(ui, repo, dest=None, **opts'
     if opts.get('graph'):
         cmdutil.checkunsupportedgraphflags([], opts)
         o = hg._outgoing(ui, repo, dest, opts)
-        if o is None:
+        if not o:
             return
 
         revdag = cmdutil.graphrevs(repo, o, opts)

mercurial/hg.py

@@ -585,7 +585,6 b' def _outgoing(ui, repo, dest, opts):'
     o = outgoing.missing
     if not o:
         scmutil.nochangesfound(repo.ui, repo, outgoing.excluded)
-        return None
     return o
 
 def outgoing(ui, repo, dest, opts):
@@ -600,7 +599,7 b' def outgoing(ui, repo, dest, opts):'
 
     limit = cmdutil.loglimit(opts)
     o = _outgoing(ui, repo, dest, opts)
-    if o is None:
+    if not o:
         return recurse()
 
     if opts.get('newest_first'):