Add null check to ExoPlayerImpl.isTunnelingEnabled

`TrackSelectorResult.rendererConfigurations` can contain null elements:
> A null entry indicates the corresponding renderer should be disabled.

This wasn't caught by the nullness checker because `ExoPlayerImpl` is
currently excluded from analysis.

#minor-release

Issue: google#10977
PiperOrigin-RevId: 508619169
(cherry picked from commit 5e3cd7a)

library/core/src/main/java/com/google/android/exoplayer2/ExoPlayerImpl.java

@@ -1713,8 +1713,9 @@ public void setDeviceMuted(boolean muted) {
   @Override
   public boolean isTunnelingEnabled() {
     verifyApplicationThread();
-    for (RendererConfiguration config : playbackInfo.trackSelectorResult.rendererConfigurations) {
-      if (config.tunneling) {
+    for (@Nullable
+    RendererConfiguration config : playbackInfo.trackSelectorResult.rendererConfigurations) {
+      if (config != null && config.tunneling) {
         return true;
       }
     }