MP4: set TrackSampleTable to 0 when there are no samples

Because the stbl atom is mandatory, there is no reason for having a
special C.TIME_UNSET value instead of 0.

PiperOrigin-RevId: 319496999

library/extractor/src/main/java/com/google/android/exoplayer2/extractor/mp4/AtomParsers.java

@@ -315,7 +315,7 @@ import org.checkerframework.checker.nullness.compatqual.NullableType;
           /* maximumSize= */ 0,
           /* timestampsUs= */ new long[0],
           /* flags= */ new int[0],
-          /* durationUs= */ C.TIME_UNSET);
+          /* durationUs= */ 0);
     }
 
     // Entries are byte offsets of chunks.

library/extractor/src/main/java/com/google/android/exoplayer2/extractor/mp4/TrackSampleTable.java

@@ -38,10 +38,7 @@ import com.google.android.exoplayer2.util.Util;
   public final long[] timestampsUs;
   /** Sample flags. */
   public final int[] flags;
-  /**
-   * The duration of the track sample table in microseconds, or {@link C#TIME_UNSET} if the sample
-   * table is empty.
-   */
+  /** The duration of the track sample table in microseconds. */
   public final long durationUs;
 
   public TrackSampleTable(