protonmsg: don't send rpm.sign messages when the sigkey is empty

When a build completes, we store the original, unsigned rpm headers
to disk. This results in the postRPMSign callback being called, and
a rpm.sign message being sent with an empty sigkey. The intended
sematics of the rpm.sign message is to notify a service when an rpm
has been signed. Therefore, we should not be sending a message when
the sigkey is empty.

plugins/hub/protonmsg.py

@@ -196,6 +196,8 @@ def prep_import(cbtype, *args, **kws):
 @convert_datetime
 @callback('postRPMSign')
 def prep_rpm_sign(cbtype, *args, **kws):
+    if not kws['sigkey']:
+        return
     address = 'sign.rpm'
     props = {'type': cbtype[4:],
              'sigkey': kws['sigkey'],