Form: 如果需要验证的fields为空，调用验证时立刻返回callback (#3944)

* Form: 如果需要验证的fields为空，调用验证时立刻返回callback
同时修复几个代码格式用来通过npm run dist

* fix travis

* fix travis

* fix travis

packages/form/src/form.vue

@@ -59,6 +59,10 @@
       validate(callback) {
         let valid = true;
         let count = 0;
+        // 如果需要验证的fields为空，调用验证时立刻返回callback
+        if (this.fields.length === 0 && callback) {
+          callback(true);
+        }
         this.fields.forEach((field, index) => {
           field.validate('', errors => {
             if (errors) {

packages/slider/src/button.vue

@@ -158,4 +158,5 @@
       }
     }
   };
-</script>
+</script>
\ No newline at end of file
+

packages/tabs/src/tab-nav.vue

@@ -172,4 +172,5 @@
       );
     }
   };
-</script>
+</script>
\ No newline at end of file
+

packages/upload/src/upload-dragger.vue

@@ -27,4 +27,5 @@
       }
     }
   };
-</script>
+</script>
\ No newline at end of file
+