Fix crash in simplifiedExpression for a null expression

autotests/parsertest.cpp

@@ -454,6 +454,8 @@ private Q_SLOTS:
 
         OpeningHours oh(expression);
         QCOMPARE(oh.error(), error);
+        (void)oh.normalizedExpression(); // don't crash
+        (void)oh.simplifiedExpression(); // don't crash
     }
 };
 

src/lib/openinghours.cpp

@@ -123,7 +123,7 @@ void OpeningHoursPrivate::autocorrect()
 
 void OpeningHoursPrivate::simplify()
 {
-    if (m_error == OpeningHours::SyntaxError) {
+    if (m_error == OpeningHours::SyntaxError || m_rules.empty()) {
         return;
     }