mon: fix mon health nil pointer exception

shouldFailoverMonImmediately has a nil pointer exception with mons on
PVC. Rook-ceph-mon-endpoint does not store node information of the mons
on PVC. So `mapping.Hostname` throws nill pointer

Signed-off-by: Santosh Pillai <sapillai@redhat.com>

pkg/operator/ceph/cluster/mon/health.go

@@ -394,6 +394,11 @@ func (c *Cluster) shouldFailoverMonImmediately(ctx context.Context, monName stri
 	logger.Debugf("checking if mon %q is scheduled on a node", monName)
 	assignedNode := ""
 	if mapping, ok := c.mapping.Schedule[monName]; ok {
+		if mapping == nil {
+			// mon schedule information is not available in the rook-ceph-mon-endpoints configmap in case of mons on PVC
+			logger.Debugf("mon schedule information is not available for mon %q", monName)
+			return false
+		}
 		assignedNode = mapping.Hostname
 	}
 	if assignedNode == "" {