fix(tooltip): allow null and undefined as values for tooltip

The documentation says that falsy values are accepted, but in strict mode, only the empty string could actually be passed.

fix #3845

src/tooltip/tooltip.spec.ts

@@ -193,6 +193,17 @@ describe('ngb-tooltip', () => {
       expect(windowEl).toBeNull();
     });
 
+    it('should not open a tooltip if content is empty', () => {
+      const fixture = createTestComponent(`<div ngbTooltip=""></div>`);
+      const directive = fixture.debugElement.query(By.directive(NgbTooltip));
+
+      triggerEvent(directive, 'mouseenter');
+      fixture.detectChanges();
+      const windowEl = getWindow(fixture.nativeElement);
+
+      expect(windowEl).toBeNull();
+    });
+
     it('should close the tooltip tooltip if content becomes falsy', () => {
       const fixture = createTestComponent(`<div [ngbTooltip]="name"></div>`);
       const directive = fixture.debugElement.query(By.directive(NgbTooltip));

src/tooltip/tooltip.ts

@@ -147,7 +147,7 @@ export class NgbTooltip implements OnInit, OnDestroy, OnChanges {
    */
   @Output() hidden = new EventEmitter();
 
-  private _ngbTooltip: string | TemplateRef<any>;
+  private _ngbTooltip: string | TemplateRef<any>| null | undefined;
   private _ngbTooltipWindowId = `ngb-tooltip-${nextId++}`;
   private _popupService: PopupService<NgbTooltipWindow>;
   private _windowRef: ComponentRef<NgbTooltipWindow>| null = null;
@@ -187,7 +187,7 @@ export class NgbTooltip implements OnInit, OnDestroy, OnChanges {
    * If the content if falsy, the tooltip won't open.
    */
   @Input()
-  set ngbTooltip(value: string | TemplateRef<any>) {
+  set ngbTooltip(value: string | TemplateRef<any>| null | undefined) {
     this._ngbTooltip = value;
     if (!value && this._windowRef) {
       this.close();