fix(recipe): support nil, null, undefined for condition field (#1091)

Because

- we need to allow the condition field to be based on a null value.

This commit

- supports `nil`, `null`, and `undefined` for the condition field

Author: donch1989

pkg/recipe/eval.go

@@ -405,6 +405,15 @@ func evalExpr(expr ast.Expr, value map[string]any) (any, error) {
 		if e.Name == "false" {
 			return false, nil
 		}
+		if e.Name == "nil" {
+			return nil, nil
+		}
+		if e.Name == "null" {
+			return nil, nil
+		}
+		if e.Name == "undefined" {
+			return nil, nil
+		}
 		val, ok := value[e.Name]
 		if !ok {
 			return nil, fmt.Errorf("undefined variable: %s", e.Name)