Avoid dif/1 attribute with empty list

Closes #1956

src/lib/dif.pl

@@ -35,7 +35,10 @@ vars_remove_goal([], _).
 vars_remove_goal([Var|Vars], Goal0) :-
     get_atts(Var, +dif(Goals0)),
     remove_goal(Goals0, Goal0, Goals),
-    put_atts(Var, +dif(Goals)),
+    (   Goals = [] ->
+        put_atts(Var, -dif(_))
+    ;   put_atts(Var, +dif(Goals))
+    ),
     vars_remove_goal(Vars, Goal0).
 
 reinforce_goal(Goal0, Goal) :-

src/tests/dif.pl

@@ -189,6 +189,13 @@ test("dif#12 but with multiple variables in the residuals",(
     Res = []
 )).
 
+% https://github.com/mthom/scryer-prolog/issues/1956
+test("scryer-prolog#1956",(
+    call_residue_vars((dif(a-a,X-_),X=b), Res),
+    X == b,
+    Res = []
+)).
+
 main :-
     findall(test(Name, Goal), test(Name, Goal), Tests),
     run_tests(Tests, Failed),