CppEditor: Fix raw string highlighting with embedded empty lines

We missed an early return in highlightBlock().

Task-number: QTCREATORBUG-29200
Change-Id: I3f32c2948ff778a9b558850235058537fcd48fd1
Reviewed-by: Qt CI Bot <qt_ci_bot@qt-project.org>
Reviewed-by: <github-actions-qt-creator@cristianadam.eu>
Reviewed-by: Fabian Kosmale <fabian.kosmale@qt.io>
Reviewed-by: David Schulz <david.schulz@qt.io>

src/plugins/cppeditor/testcases/highlightingtestcase.cpp

@@ -1,7 +1,7 @@
 auto func()
 {
     return R"(foo
-    foobar
+
         R"notaprefix!(
     barfoobar)" R"(second)" /* comment */ R"(third)";
 }