Make existing empty metadata properties optional

Editing of snapshot image or updating its metadata from Horizon is
not allowed until existing properties with empty values are set or
removed from metadata tree.

They are allowed to be None, therefore Horizon should consider them
optional too.

Closes-Bug: #1945706
Change-Id: Idfec71d209430c7d069133a40ca1a7d340163783

horizon/static/framework/widgets/metadata/tree/metadata-tree-item.html

@@ -13,7 +13,7 @@
            type="text"
            class="form-control"
            ng-pattern="ctrl.item.leaf.pattern"
-           required
+           ng-required="ctrl.item.leaf.required"
            ng-model="ctrl.item.leaf.value"
            ng-minlength="{$ ::ctrl.item.leaf.minLength $}"
            ng-maxlength="{$ ::ctrl.item.leaf.maxLength $}"

horizon/static/framework/widgets/metadata/tree/tree.service.js

@@ -56,6 +56,7 @@
       this.default = null;
       this.type = 'string';
       this.readonly = false;
+      this.required = true;
       this.operators = ['<in>'];
       angular.extend(this, json);
       this.operator = this.operators[0];
@@ -73,6 +74,11 @@
         return;
       }
 
+      // if the existing property has empty string value, make the field not required
+      if (value === '') {
+        this.required = false;
+      }
+
       switch (this.type) {
         case 'integer':
           this.value = parseInt(value, 10);