Fix a serialization issue

Our data model allows null sum_types, but createrepo_c types don't
allow setting such values. It defaults to a value of 0, or "unknown"

[noissue]

pulp_rpm/app/models/advisory.py

@@ -470,6 +470,7 @@ def to_createrepo_c(self):
             # If self.sum_type is 0 the result will be createrepo_c printing "Unknown checksum",
             # an invalid value, thus we don't want to set self.sum if this will happen.
             pkg.sum = self.sum
+        if self.sum_type:
             pkg.sum_type = self.sum_type
 
         return pkg