fix: accessibility issue on video transcripts (#37587)

This commit fixes accessibility issue for video transcripts as when a
video component's SRT file has an empty line, the transcript has an
empty link that is still interactive.

Empty links should not be interactive and should be hidden from
keyboard users as it takes extra click for them when an empty line
occurs in the transcript.

Co-authored-by: Muhammad Faraz  Maqsood <[email protected]>

Author: Faraz32123

xmodule/static/css-builtin-blocks/VideoBlockDisplay.css

@@ -708,6 +708,10 @@
     line-height: lh();
 }
 
+.xmodule_display.xmodule_VideoBlock .video .subtitles .subtitles-menu li:has(> span:empty) {
+  display: none;
+}
+
 .xmodule_display.xmodule_VideoBlock .video .subtitles .subtitles-menu li span {
     display: block;
 }