QLineEdit: Fix behavior of DeleteStartOfWord key

DeleteStartOfWord key behaves like Delete key when the cursor
is at the beginning of the line. Do nothing instead.

Task-number: QTBUG-65855
Change-Id: I56402961b44ec6fe3a382df68a76db36891c4522
Reviewed-by: Richard Moe Gustavsen <richard.gustavsen@qt.io>

src/widgets/widgets/qwidgetlinecontrol.cpp

@@ -1840,7 +1840,8 @@ void QWidgetLineControl::processKeyEvent(QKeyEvent* event)
     else if (event == QKeySequence::DeleteStartOfWord) {
         if (!isReadOnly()) {
             cursorWordBackward(true);
-            del();
+            if (hasSelectedText())
+                del();
         }
     } else if (event == QKeySequence::DeleteCompleteLine) {
         if (!isReadOnly()) {