fix(module:tabs): tabs add btn disappear after all tab closed (#7076)

* fix: fix Tabs  adding & closable  error

There is a problem with the tabs component. When opening and closing the tab page and the Add button, if all the open tab pages are closed, the Add button will not be found. This problem can be repeated in the official website document demo.

Tabs组件有一个问题，当开启关闭tab页和添加按钮时，如果把开启的tab页全部关掉，添加按钮也找不到了，这个问题在官网文档demo中即可复现。

https://ng.ant.design/components/tabs/en#components-tabs-demo-card-top

* fix(module:tabs): tabs add btn disappear after all tab closed

1.Move the add btn height css to patch.less from index.less
2.Add the btn shown test

close #7077

components/tabs/style/patch.less

@@ -89,4 +89,11 @@ nz-tabs-nav {
     pointer-events: none;
     color: @disabled-color;
   }
+  > .@{tab-prefix-cls}-nav,
+  > div > .@{tab-prefix-cls}-nav {
+
+    .@{tab-prefix-cls}-nav-add {
+      min-height: @tabs-card-height;
+    }
+  }
 }

components/tabs/tabset.component.spec.ts

@@ -586,6 +586,17 @@ describe('NzTabSet', () => {
 
       expect(fixture.componentInstance.handleSelection).not.toHaveBeenCalled();
     }));
+
+    it('should show add btn after all tabs are removed', () => {
+      const component = fixture.debugElement.componentInstance;
+      component.closable = true;
+      component.type = 'editable-card';
+      fixture.detectChanges();
+      fixture.componentInstance.tabs.splice(0, component.tabs.length);
+      fixture.detectChanges();
+      const btnCount = fixture.debugElement.queryAll(By.css('.ant-tabs-nav-add')).length;
+      expect(btnCount).toBeGreaterThan(0);
+    });
   });
 
   describe('async tabs', () => {
@@ -987,7 +998,11 @@ class DisableTabsTestComponent {
 
 @Component({
   template: `
-    <nz-tabset [(nzSelectedIndex)]="selectedIndex" (nzSelectedIndexChange)="handleSelection($event)">
+    <nz-tabset
+      [(nzSelectedIndex)]="selectedIndex"
+      [nzType]="'editable-card'"
+      (nzSelectedIndexChange)="handleSelection($event)"
+    >
       <nz-tab *ngFor="let tab of tabs" [nzTitle]="tab.title">
         {{ tab.content }}
       </nz-tab>

components/tabs/tabset.component.ts

@@ -64,7 +64,7 @@ let nextId = 0;
   ],
   template: `
     <nz-tabs-nav
-      *ngIf="tabs.length"
+      *ngIf="tabs.length || addable"
       [ngStyle]="nzTabBarStyle"
       [selectedIndex]="nzSelectedIndex || 0"
       [inkBarAnimated]="inkBarAnimated"