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"

Let's only print a checksum if we have a sensible value for both sum and
sum_type.

[noissue]

pulp_rpm/app/models/advisory.py

@@ -466,7 +466,10 @@ def to_createrepo_c(self):
             pkg.relogin_suggested = self.relogin_suggested
         if self.restart_suggested:
             pkg.restart_suggested = self.restart_suggested
-        if self.sum:
+        if self.sum and self.sum_type:
+            # Whether self.sum is None controls whether or not the <sum> tag will be serialized
+            # 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
             pkg.sum_type = self.sum_type