fix: Make tooltips fade out

[hsource]

• Bug fix
• New feature
• Chore
• Breaking change
• There is an open issue which this change addresses
• I have read the CONTRIBUTING document.
• My commits follow the Git Commit Guidelines
• My code follows the code style of this project.
• My change requires a change to the documentation.
  • I have updated the documentation accordingly.
• My change requires a change to Typescript typings.
  • I have updated the typings accordingly.
• I have added tests to cover my changes.
• All new and existing tests passed.

Tooltips currently don't actually fade out, because as soon as isOpen is set to false, the render function in TooltipPopoverWrapper returns null.

This change makes it so that it returns something in the render function even when isOpen is false. We still only call this.updateTarget() when the tooltip is open. This ensures that the component still works for tooltips that open on a target appearing only after initial page rendering.

Before	After
https://user-images.githubusercontent.com/2937410/102849462-0fa82800-43cc-11eb-92bb-b8f1d7a7443b.mov	https://user-images.githubusercontent.com/2937410/102849480-1767cc80-43cc-11eb-8030-40cab32414ed.mov

[kyletsang]

Thanks for the PRs!