Allow the != operator to be used on nil

expr.go

@@ -55,13 +55,17 @@ func (t *Template) performOp(a, b reflect.Value, op ast.Operator) (any, error) {
 			return nil, ast.PosError(op, "the in operator can only be used on strings, arrays, and slices (got %s and %s)", a.Type(), b.Type())
 		}
 	} else if !b.IsValid() {
-		if op.Value != "==" {
-			return nil, ast.PosError(op, "invalid operator for nil value (expected ==, got %s)", op.Value)
+		if op.Value != "==" && op.Value != "!=" {
+			return nil, ast.PosError(op, "invalid operator for nil value (expected == or !=, got %s)", op.Value)
 		}
 
 		switch a.Kind() {
 		case reflect.Chan, reflect.Slice, reflect.Map, reflect.Func, reflect.Interface, reflect.Pointer:
-			return a.IsNil(), nil
+			if op.Value == "==" {
+				return a.IsNil(), nil
+			} else {
+				return !a.IsNil(), nil
+			}
 		default:
 			return nil, ast.PosError(op, "values of type %s cannot be compared against nil", a.Type())
 		}