models: Handle unset 'Submission.content'

It's unlikely but 'Submission.content' is nullable and we weren't
handling this here. Fix that.

Signed-off-by: Stephen Finucane <stephen@that.guru>
Fixes: 7f6685a ("Fix CRLF newlines upon submission changes")

Author: stephenfin

patchwork/models.py

@@ -347,7 +347,9 @@ def save(self, *args, **kwargs):
         # message content to '\r\n'. We need to fix them to avoid problems,
         # especially as git complains about malformed patches when PW runs
         # on PY2
-        self.content = self.content.replace('\r\n', '\n')
+        if self.content:
+            self.content = self.content.replace('\r\n', '\n')
+
         super(EmailMixin, self).save(*args, **kwargs)
 
     class Meta: