Merge pull request #940 from tneotia/bugfix/empty-video-poster

Fix exception on empty video poster

Author: erickok

lib/src/replaced_element.dart

@@ -181,7 +181,7 @@ class VideoContentElement extends ReplacedElement {
             videoPlayerController: VideoPlayerController.network(
               src.first ?? "",
             ),
-            placeholder: poster != null
+            placeholder: poster != null && poster!.isNotEmpty
                 ? Image.network(poster!)
                 : Container(color: Colors.black),
             autoPlay: autoplay,