Avoid AttributeError on simple connection and disconnection

Without this patch, this simple script for miltertest:

----
conn = mt.connect("unix:milter.sock")
if conn == nil then
  error "mt.connect() failed"
end
if mt.conninfo(conn, nil, "unspec") ~= nil then
  error "mt.conninfo() failed"
end
if mt.getreply(conn) ~= SMFIR_CONTINUE then
     error "mt.conninfo() unexpected reply"
end
mt.disconnect(conn)
----

Produces the following error:

Traceback (most recent call last):
  File "/usr/lib/python2.7/dist-packages/Milter/__init__.py", line 702, in connect_callback
    return m.connect(hostname,family,hostaddr)
  File "/usr/lib/python2.7/dist-packages/Milter/__init__.py", line 173, in wrapper
    rc = func(self,*args)
  File "/home/dkg/src/dkimpy-milter/dkimpy-milter/dkimpy_milter/__init__.py", line 64, in connect
    self.receiver = self.getsymval('j').strip()
AttributeError: 'NoneType' object has no attribute 'strip'

dkimpy_milter/__init__.py

@@ -61,7 +61,9 @@ class dkimMilter(Milter.Base):
         self.external_connection = False
         self.hello_name = None
         # sometimes people put extra space in sendmail config, so we strip
-        self.receiver = self.getsymval('j').strip()
+        self.receiver = self.getsymval('j')
+        if self.receiver is not None:
+            self.receiver = self.receiver.strip()
         try:
             self.AuthservID = milterconfig['AuthservID']
         except: