Let MediaSourceFactory setDrmSessionManager accept null

Issue: #7168

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

@@ -628,7 +628,10 @@ public class PlayerActivity extends AppCompatActivity
 
             @Override
             public MediaSourceFactory setDrmSessionManager(DrmSessionManager<?> drmSessionManager) {
-              this.drmSessionManager = drmSessionManager;
+              this.drmSessionManager =
+                  drmSessionManager != null
+                      ? drmSessionManager
+                      : DrmSessionManager.getDummyDrmSessionManager();
               return this;
             }
 

library/core/src/main/java/com/google/android/exoplayer2/source/ProgressiveMediaSource.java

@@ -174,7 +174,10 @@ public final class ProgressiveMediaSource extends BaseMediaSource
     @Override
     public Factory setDrmSessionManager(DrmSessionManager<?> drmSessionManager) {
       Assertions.checkState(!isCreateCalled);
-      this.drmSessionManager = drmSessionManager;
+      this.drmSessionManager =
+          drmSessionManager != null
+              ? drmSessionManager
+              : DrmSessionManager.getDummyDrmSessionManager();
       return this;
     }
 

library/dash/src/main/java/com/google/android/exoplayer2/source/dash/DashMediaSource.java

@@ -307,7 +307,10 @@ public final class DashMediaSource extends BaseMediaSource {
     @Override
     public Factory setDrmSessionManager(DrmSessionManager<?> drmSessionManager) {
       Assertions.checkState(!isCreateCalled);
-      this.drmSessionManager = drmSessionManager;
+      this.drmSessionManager =
+          drmSessionManager != null
+              ? drmSessionManager
+              : DrmSessionManager.getDummyDrmSessionManager();
       return this;
     }
 

library/hls/src/main/java/com/google/android/exoplayer2/source/hls/HlsMediaSource.java

@@ -323,7 +323,10 @@ public final class HlsMediaSource extends BaseMediaSource
     @Override
     public Factory setDrmSessionManager(DrmSessionManager<?> drmSessionManager) {
       Assertions.checkState(!isCreateCalled);
-      this.drmSessionManager = drmSessionManager;
+      this.drmSessionManager =
+          drmSessionManager != null
+              ? drmSessionManager
+              : DrmSessionManager.getDummyDrmSessionManager();
       return this;
     }
 

library/smoothstreaming/src/main/java/com/google/android/exoplayer2/source/smoothstreaming/SsMediaSource.java

@@ -273,7 +273,10 @@ public final class SsMediaSource extends BaseMediaSource
     @Override
     public Factory setDrmSessionManager(DrmSessionManager<?> drmSessionManager) {
       Assertions.checkState(!isCreateCalled);
-      this.drmSessionManager = drmSessionManager;
+      this.drmSessionManager =
+          drmSessionManager != null
+              ? drmSessionManager
+              : DrmSessionManager.getDummyDrmSessionManager();
       return this;
     }