Skip to-many check in Equal if both lengths are zero

reflect.DeepEqual returns false if a nil slice is compared to an
empty slice. For the purpose of the Equal function, both a nil and
an empty slice are considered equal which is not an opinion that
the reflect.DeepEqual function shares.

resource.go

@@ -115,8 +115,12 @@ func Equal(r1, r2 Resource) bool {
 				return false
 			}
 		} else {
-			if !reflect.DeepEqual(r1.GetToMany(rel1.Name), r2.GetToMany(rel2.Name)) {
-				return false
+			v1 := r1.GetToMany(rel1.Name)
+			v2 := r2.GetToMany(rel2.Name)
+			if len(v1) != 0 || len(v2) != 0 {
+				if !reflect.DeepEqual(v1, v2) {
+					return false
+				}
 			}
 		}
 	}