patman: Correct pylint errors

Fix pylint errors that can be fixed and mask those that seem to be
incorrect.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/tools/patman/commit.py b/tools/patman/commit.py
index c331a3b..a645b22 100644
--- a/tools/patman/commit.py
+++ b/tools/patman/commit.py
@@ -31,7 +31,7 @@
     """
     def __init__(self, hash):
         self.hash = hash
-        self.subject = None
+        self.subject = ''
         self.tags = []
         self.changes = {}
         self.cc_list = []