[#7021] Fixed error serializing programmatically created commits

Author: johnsca

git/objects/commit.py

@@ -121,7 +121,7 @@ def __init__(self, repo, binsha, tree=None, author=None, authored_date=None, aut
 			self.parents = parents
 		if encoding is not None:
 			self.encoding = encoding
-		if gpgsig is not None:
+		if binsha == '\x00'*20 or gpgsig is not None:
 			self.gpgsig = gpgsig
 
 	@classmethod