fix(Field): should not reset validation after blured (#8409)

* fix(Field): should not reset validation after blured

* test(Form): add reset validation test case

src/field/Field.tsx

@@ -238,7 +238,9 @@ export default defineComponent({
           return defaultTrigger;
         });
 
-        validate(rules);
+        if (rules.length) {
+          validate(rules);
+        }
       }
     };
 

src/form/test/index.spec.tsx

@@ -0,0 +1,36 @@
+import { Form } from '..';
+import { Field } from '../../field';
+import { mountForm } from './shared';
+import { later } from '../../../test';
+
+test('should not reset validation after blured when validate-trigger is onChange', async () => {
+  const validator = (val: string) => val.length > 4;
+  const wrapper = mountForm({
+    data() {
+      return {
+        value: '',
+      };
+    },
+    render() {
+      return (
+        <Form validateTrigger="onChange">
+          <Field
+            v-model={this.value}
+            name="username"
+            rules={[{ validator, message: 'msg' }]}
+          />
+        </Form>
+      );
+    },
+  });
+
+  const input = wrapper.find('input');
+  input.element.value = '1';
+  await input.trigger('input');
+  await later();
+
+  expect(wrapper.find('.van-field__error-message').exists()).toBeTruthy();
+
+  await input.trigger('blur');
+  expect(wrapper.find('.van-field__error-message').exists()).toBeTruthy();
+});