empty expressions no more return error, now they return nil

ast.go

@@ -5,7 +5,6 @@
 package expr
 
 import (
-	"errors"
 	"strings"
 )
 
@@ -129,8 +128,8 @@ func (self *ast) eval(ctx ExprContext, preset bool) (result any, err error) {
 		if err == nil {
 			result, err = self.root.compute(ctx)
 		}
-	} else {
-		err = errors.New("empty expression")
+		// } else {
+		// 	err = errors.New("empty expression")
 	}
 	return
 }