fix reverse fill issue

Author: dipaksarkar

docs/src/App.vue

@@ -1,5 +1,8 @@
 <template>
-  <vue-number v-bind="config" v-model="price" />
+  <div>
+    <vue-number v-bind="config" v-model="price" />
+    {{ price }}
+  </div>
 </template>
 
 <script>
@@ -10,6 +13,7 @@ export default {
       config: {
         prefix: '$',
         // min: 0,
+        reverseFill: true
       }
     }
   },

src/number-format.ts

@@ -163,7 +163,10 @@ export default class NumberFormat {
    */
   unformat(input: Input): string {
     this.input = input
-    if (this.isNull() && !this.options.reverseFill) {
+    if (this.isNull()) {
+      return this.options.nullValue
+    }
+    if (this.options.reverseFill && this.realNumber() <= 0) {
       return this.options.nullValue
     }
     return this.sign() + this.realNumber()