Fix a crash on pane close (#17886)

The underlying issue is that the "Pane" is used both as a model and as
a UI element and so a pane loses its content as soon as it is closed,
but the tree only gets reordered after the animation has finished.
This PR is truly just a hotfix, because it doesn't solve this issue,
it only adds checks to the function that crashes.

Closes #17869
Closes #17871

## Validation Steps Performed
* `Split pane` a few times
* Run the "Close all other panes" action
* Doesn't crash ✅

(cherry picked from commit bc6f3e2)
Service-Card-Id: PVTI_lADOAF3p4s4AmhmszgTEJYE
Service-Version: 1.21

src/cascadia/TerminalApp/Pane.cpp

@@ -2955,13 +2955,13 @@ bool Pane::ContainsReadOnly() const
 // - <none>
 void Pane::CollectTaskbarStates(std::vector<winrt::TerminalApp::TaskbarState>& states)
 {
-    if (_IsLeaf())
+    if (_content)
     {
         auto tbState{ winrt::make<winrt::TerminalApp::implementation::TaskbarState>(_content.TaskbarState(),
                                                                                     _content.TaskbarProgress()) };
         states.push_back(tbState);
     }
-    else
+    else if (_firstChild && _secondChild)
     {
         _firstChild->CollectTaskbarStates(states);
         _secondChild->CollectTaskbarStates(states);