tooltip is not hidden when setting value to null

[sasensi]

Reproduction link

https://stackblitz.com/edit/ng-zorro-antd-issue-tooltip

Steps to reproduce

1 - hover tooltip area and see that tooltip is displayed
2 - click on "toggle tooltip visibility" button
3 - hover tooltip area and see that tooltip is still displayed

What is expected?

tooltip should no longer be displayed after switching the nz-tooltip value to null

What is actually happening?

tooltip is still displayed after switching the nz-tooltip value to null

Environment	Info
ng-zorro-antd	8.4.1
Browser	chrome

---

My reproduction link also provides a workaround relying on nzOverlayStyle to force the tooltip to be hidden.
But I think that this is not an ideal solution.

[wzhudev]

<div class="case">
    <h1>Issue</h1>
    <div
            class="tooltip-area"
            nz-tooltip
            [nzTitle]="tooltipVisible ? 'here is the tooltip' : ''"
    >
        tooltip should be {{tooltipVisible ? 'visible' : 'hidden'}}
    </div>
    <button nz-button (click)="tooltipVisible=!tooltipVisible">
        toggle tooltip visibility
    </button>
</div>

You can use it as a workaround now.

[vthinkxie]

close via #4392