git p4: Ignore P4EDITOR if it is empty

p4 itself treats an empty value for P4EDITOR as the same as
having P4EDITOR unset. Do the same for "git p4".

Signed-off-by: Luke Diamand <luke@diamand.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

git-p4.py

@@ -994,7 +994,7 @@ class P4Submit(Command, P4UserMap):
         mtime = os.stat(template_file).st_mtime
 
         # invoke the editor
-        if os.environ.has_key("P4EDITOR"):
+        if os.environ.has_key("P4EDITOR") and (os.environ.get("P4EDITOR") != ""):
             editor = os.environ.get("P4EDITOR")
         else:
             editor = read_pipe("git var GIT_EDITOR").strip()