gstreamer: Replace NULL with nullptr

Usage of NULL has slowly crept in the libcamerasrc sources. Replace it
with nullptr.

Reported-by: Nicolas Dufresne <nicolas.dufresne@collabora.com>
Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Barnabás Pőcze <barnabas.pocze@ideasonboard.com>
Reviewed-by: Nicolas Dufresne <nicolas.dufresne@collabora.com>

src/gstreamer/gstlibcamera-controls.cpp.in

@@ -68,7 +68,7 @@ static const GEnumValue {{ ctrl.name|snake_case }}_types[] = {
 		"{{ enum.gst_name }}"
 	},
 {%- endfor %}
-	{0, NULL, NULL}
+	{0, nullptr, nullptr}
 };
 
 #define TYPE_{{ ctrl.name|snake_case|upper }} \