git-p4: return an empty list if a list config has no values

Signed-off-by: Lars Schneider <larsxschneider@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

git-p4.py

@@ -638,6 +638,8 @@ def gitConfigList(key):
     if not _gitConfig.has_key(key):
         s = read_pipe(["git", "config", "--get-all", key], ignore_error=True)
         _gitConfig[key] = s.strip().split(os.linesep)
+        if _gitConfig[key] == ['']:
+            _gitConfig[key] = []
     return _gitConfig[key]
 
 def p4BranchesInGit(branchesAreInRemotes=True):