Fix crash when closing multiple panes simultaneously

[carlos-zamora]

Summary of the Pull Request

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

[lhecker]

Should we still call _setPaneContent if _takePaneContent returned null?

(Thanks for fixing this btw!)

[carlos-zamora]

terminal/src/cascadia/TerminalApp/Pane.cpp

Lines 1720 to 1734 in ad14922

	void Pane::_setPaneContent(IPaneContent content)
	{
	// The IPaneContent::Close() implementation may be buggy and raise the CloseRequested event again.
	// _takePaneContent() avoids this as it revokes the event handler.
	if (const auto c = _takePaneContent())
	{
	c.Close();
	}
	if (content)
	{
	_content = std::move(content);
	_closeRequestedRevoker = _content.CloseRequested(winrt::auto_revoke, [this](auto&&, auto&&) { Close(); });
	}
	}

The whole function results in a no-op, so we can totally split this up into:

const auto& content = remainingChild->_takePaneContent();
if (!content)
{
   Closed.raise(nullptr, nullptr);
   return;
}
_setPaneContent(content);

Up to you. I'm indifferent to it, personally.