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

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

Fixes #3886

lib/media/streaming_engine.js

@@ -1644,7 +1644,7 @@ 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)) {
       // See DASH sec. 5.10.4.1
       // A special scheme in DASH used to signal manifest updates.
       if (schemeId == 'urn:mpeg:dash:event:2012') {