Listeners macro: skip on empty mine

Don't fail if mine does not contain information about the queried
minion.
In the future it would be nice to add another conditional to allow such
minions to fall-back to the locally executed network module for
masterless setups.

Signed-off-by: Georg Pfuetzenreuter <mail@georg-pfuetzenreuter.net>

pillar/role/salt/common.sls

@@ -1,4 +1,3 @@
 salt:
   hash_type: sha512
   log_level: info
-  rootgroup: salt

pillar/role/salt/master.sls

@@ -78,3 +78,4 @@ salt:
     auth.ldap.binddn: ${'secret_salt:master:auth.ldap.binddn'}
     auth.ldap.bindpw: ${'secret_salt:master:auth.ldap.bindpw'}
     auth.ldap.filter: ${'secret_salt:master:auth.ldap.filter'}
+    rootgroup: salt