Demo: Fix NPE in SampleChooserActivity

This prevents the demo app loading media.exolist.json. The exception
was introduced by
https://github.com/google/ExoPlayer/commit/8fd1381a84470813827a83921cf3dd2b8603ecb0

PiperOrigin-RevId: 403973062

demos/main/src/main/java/com/google/android/exoplayer2/demo/SampleChooserActivity.java

@@ -348,7 +348,7 @@ public class SampleChooserActivity extends AppCompatActivity
       String subtitleLanguage = null;
       UUID drmUuid = null;
       String drmLicenseUri = null;
-      ImmutableMap<String, String> drmLicenseRequestHeaders = null;
+      ImmutableMap<String, String> drmLicenseRequestHeaders = ImmutableMap.of();
       boolean drmSessionForClearContent = false;
       boolean drmMultiSession = false;
       boolean drmForceDefaultLicenseUri = false;
@@ -453,7 +453,7 @@ public class SampleChooserActivity extends AppCompatActivity
         } else {
           checkState(drmLicenseUri == null, "drm_uuid is required if drm_license_uri is set.");
           checkState(
-              drmLicenseRequestHeaders == null,
+              drmLicenseRequestHeaders.isEmpty(),
               "drm_uuid is required if drm_key_request_properties is set.");
           checkState(
               !drmSessionForClearContent,