lang/funcs: Don't panic when distinct called with empty list

lang/funcs/collection.go

@@ -363,6 +363,9 @@ var DistinctFunc = function.New(&function.Spec{
 			}
 		}
 
+		if len(list) == 0 {
+			return cty.ListValEmpty(retType.ElementType()), nil
+		}
 		return cty.ListVal(list), nil
 	},
 })

lang/funcs/collection_test.go

@@ -901,6 +901,11 @@ func TestDistinct(t *testing.T) {
 			}),
 			false,
 		},
+		{
+			cty.ListValEmpty(cty.String),
+			cty.ListValEmpty(cty.String),
+			false,
+		},
 		{
 			cty.ListVal([]cty.Value{
 				cty.StringVal("a"),