Revert "Select: set value to null when cleared (#14322) (#14650)" (#15447)

Author: iamkun

packages/select/src/select.vue

@@ -750,7 +750,7 @@
 
       deleteSelected(event) {
         event.stopPropagation();
-        const value = this.multiple ? [] : null;
+        const value = this.multiple ? [] : '';
         this.$emit('input', value);
         this.emitChange(value);
         this.visible = false;

test/unit/specs/select.spec.js

@@ -288,7 +288,7 @@ describe('Select', () => {
       const iconClear = vm.$el.querySelector('.el-input__icon.el-icon-circle-close');
       expect(iconClear).to.exist;
       iconClear.click();
-      expect(vm.value).to.equal(null);
+      expect(vm.value).to.equal('');
       done();
     }, 100);
   });