Prevent empty syncs (#922)

There is a possibility to pass nil as one of the specs and an empty spec
into syncConnectionPooler. In this case it will perfom a syncronization
because nil != empty struct. Avoid such cases and make it testable by
returning list of syncronization reasons on top together with the final
error.

pkg/cluster/cluster.go

@@ -741,7 +741,8 @@ func (c *Cluster) Update(oldSpec, newSpec *acidv1.Postgresql) error {
 	}
 
 	// sync connection pooler
-	if err := c.syncConnectionPooler(oldSpec, newSpec, c.installLookupFunction); err != nil {
+	if _, err := c.syncConnectionPooler(oldSpec, newSpec,
+		c.installLookupFunction); err != nil {
 		return fmt.Errorf("could not sync connection pooler: %v", err)
 	}