macOS: Ensure we provide a preedit format for non-attributed strings

The logic was not taking into account that we were always adding
an QInputMethodEvent::Cursor attribute, so we would never hit the
fallback code.

Pick-to: 6.2
Change-Id: I25bd725025152241a55e3fea60444c20e27db4f9
Reviewed-by: Volker Hilsheimer <volker.hilsheimer@qt.io>

src/plugins/platforms/cocoa/qnsview_complextext.mm

@@ -159,9 +159,7 @@
             }
             index = effectiveRange.location + effectiveRange.length;
         }
-    }
-
-    if (attrs.isEmpty()) {
+    } else {
         QTextCharFormat format;
         format.setFontUnderline(true);
         attrs<<QInputMethodEvent::Attribute(QInputMethodEvent::TextFormat,