fix: Fix exception in StreamingEngine for EMSG with HLS (shaka-project#3887)

Add null check for emsgSchemeIdUris when EMSG boxes are parsed, as this value is null for HLS streams

Fixes shaka-project#3886

Author: pcruiksh

lib/media/streaming_engine.js

@@ -1672,7 +1672,7 @@ shaka.media.StreamingEngine = class {
     // See DASH sec. 5.10.3.3.1
     // If a DASH client detects an event message box with a scheme that is not
     // defined in MPD, the client is expected to ignore it.
-    if (emsgSchemeIdUris.includes(schemeId) ||
+    if ((emsgSchemeIdUris && emsgSchemeIdUris.includes(schemeId)) ||
         this.config_.dispatchAllEmsgBoxes) {
       // See DASH sec. 5.10.4.1
       // A special scheme in DASH used to signal manifest updates.