Avoid processing invalid input in Contact List example

Originally, the onAccepted handler was designed to
simply emit the finished signal whenever the OK button was
clicked, without any checks. This meant that the dialog would
proceed even if some fields were left blank. Now we just
check that the strings are not empty before emitting.

Fixes: QTBUG-119147
Pick-to: 6.5 6.6
Change-Id: Ie8d3a711e139ac824a45cb5c6cfd39248a7bf117
Reviewed-by: Shawn Rutledge <shawn.rutledge@qt.io>

examples/quickcontrols/contactlist/ContactDialog.qml

@@ -41,5 +41,9 @@ Dialog {
         id: form
     }
 
-    onAccepted: finished(form.fullName.text, form.address.text, form.city.text, form.number.text)
+    onAccepted: {
+        if (form.fullName.text && form.address.text && form.city.text && form.number.text) {
+            finished(form.fullName.text, form.address.text, form.city.text, form.number.text);
+        }
+    }
 }