Fix reset.

tp-dropdown.js

@@ -413,6 +413,8 @@ class TpDropdown extends BaseElement {
     const idx = this.items.findIndex(item => item.value === this.value);
     if (idx > -1) {
       this.label = this.items[idx].label;
+    } else {
+      this.label = null;
     }
 
     this.invalid = false;
@@ -575,11 +577,7 @@ class TpDropdown extends BaseElement {
     this.invalid = false;
     this._memorizedValue = undefined;
     this._filterTerm = '';
-    if (this.default !== undefined) {
-      this.value = this.default;
-    } else {
-      this.value = undefined;
-    }
+    this.value = undefined;
   }
 
   _itemsChanged() {