Fix crash when closing multiple panes simultaneously (#19023)

Fixes a crash when multiple panes were closed simultaneously (i.e. using
broadcast input).

The root cause of this crash was that we would get a null pointer
exception when trying to access a member/function off of a null
`_content`. This is because `Pane::_CloseChild()` would always pass over
the content from the non-closed pane and attempt to hook everything up
to it.

The fix was to operate similarly to `Pane::Close()` and raise a `Closed`
event and return early. Since there's no alternative content to attach
to, might as well just close the entire pane. This propagates up the
stack of listeners to update the UI appropriately and close the parent
pane and eventually the entire tab, if necessary.
 
Closes #18071
Closes #17432

## Validation Steps Performed
1. Open 2 panes
2. Use broadcast input to send "exit" to both panes
3. ✅ Terminal doesn't crash and the tab closes gracefully

src/cascadia/TerminalApp/Pane.cpp

@@ -1414,6 +1414,13 @@ void Pane::_CloseChild(const bool closeFirst)
 
         // take the control, profile, id and isDefTermSession of the pane that _wasn't_ closed.
         _setPaneContent(remainingChild->_takePaneContent());
+        if (!_content)
+        {
+            // GH#18071: our content is still null after taking the other pane's content,
+            //           so just notify our parent that we're closed.
+            Closed.raise(nullptr, nullptr);
+            return;
+        }
         _id = remainingChild->Id();
 
         // Revoke the old event handlers. Remove both the handlers for the panes