Fix #7701 with "noSelectionUnset" QTree can't emit null

[super-seo]

Hi!
I changed the checks a bit, going back to my previous version (#12012). Because in your version it was impossible emit null (nothing happens if you click on the root element with key = null, after any other selected. if you click any element - you can't click back the root).

My if-else check for two options.
First, the old behavior always emits the value, as in the old logic, if there is no new option (when noSelectionUnset = false).
Otherwise, emit any value only if it has changed (when noSelectionUnset = true, new behavior).

What kind of change does this PR introduce?

• [*] Bugfix
• Feature
• Documentation
• Code style update
• Refactor
• Build-related changes
• Other, please describe:

Does this PR introduce a breaking change?

• Yes
• [*] No

The PR fulfills these requirements:

• [*] It's submitted to the dev branch (or v[X] branch)
• [*] When resolving a specific issue, it's referenced in the PR's title (e.g. fix: #xxx[,#xxx], where "xxx" is the issue number)
• It's been tested on a Cordova (iOS, Android) app
• It's been tested on a Electron app
• Any necessary documentation has been added or updated in the docs or explained in the PR's description.

If adding a new feature, the PR's description includes:

• A convincing reason for adding this feature (to avoid wasting your time, it's best to start a new feature discussion first and wait for approval before working on it)