gstplay: avoid getting property of playbin2 if subtitle_sid is null
requested to merge elliot_chen/gstreamer:avoid-getting-property-of-playbin2-if-subtitle_sid-is-null-when-playing-with-playbin3 into main
- There is a probability of getting "current-text" property when play with playbin3, and this property is available only in playbin2.
- It will print warning message for this case.