Preserve blank MU* output lines in WorldSession

[Copilot]

Consecutive newlines from the server were collapsing in the client output path, so intentional blank lines were not rendered. This change ensures empty completed lines are preserved as explicit empty StyledLine entries.

• Output-path behavior change

  • Updated WorldSession.OnOutputReceived (non-prompt path) to detect when parser/feed+flush emits no line for an output event.
  • In that case, it now emits StyledLine.Empty so blank server lines are retained in scrollback/output events.

• Regression coverage

  • Added EmptyOutputLine_IsAppendedAsBlankLine in WorldSessionTests.
  • Asserts an empty telnet output event increases scrollback count and appends an empty line at the tail.

var emitted = false;
foreach (var completed in _parser.Feed(e.Text))
{
    ProcessOutputLine(completed);
    emitted = true;
}

var tail = _parser.Flush();
if (tail is not null)
{
    ProcessOutputLine(tail);
    emitted = true;
}
else if (!emitted)
{
    ProcessOutputLine(StyledLine.Empty);
}

• Fixes Empty lines are not empty #17

[HarryCordewener]

@copilot An example of what a game can send, for confirmation.

Line one

This line three, previous line was empty, this is valid output. It came from the same received frame.

[Copilot]

@copilot An example of what a game can send, for confirmation.

...

Confirmed and now covered by a regression test for that exact same-frame payload shape (Line one\n\nLine three): commit 44bfbf8.