Prevent NPE in ImaAdsLoader onPositionDiscontinuity.

Any seek before the first timeline becomes available will result in a NPE.
Change it to handle that case gracefully.

Issue:#5831
PiperOrigin-RevId: 264603061

extensions/ima/src/main/java/com/google/android/exoplayer2/ext/ima/ImaAdsLoader.java

@@ -484,6 +484,7 @@ public final class ImaAdsLoader
     pendingContentPositionMs = C.TIME_UNSET;
     adGroupIndex = C.INDEX_UNSET;
     contentDurationMs = C.TIME_UNSET;
+    timeline = Timeline.EMPTY;
   }
 
   /**
@@ -967,7 +968,7 @@ public final class ImaAdsLoader
     if (contentDurationUs != C.TIME_UNSET) {
       adPlaybackState = adPlaybackState.withContentDurationUs(contentDurationUs);
     }
-    updateImaStateForPlayerState();
+    onPositionDiscontinuity(Player.DISCONTINUITY_REASON_INTERNAL);
   }
 
   @Override
@@ -1022,7 +1023,7 @@ public final class ImaAdsLoader
           }
         }
         updateAdPlaybackState();
-      } else {
+      } else if (!timeline.isEmpty()) {
         long positionMs = player.getCurrentPosition();
         timeline.getPeriod(0, period);
         int newAdGroupIndex = period.getAdGroupIndexForPositionUs(C.msToUs(positionMs));
@@ -1034,9 +1035,8 @@ public final class ImaAdsLoader
           }
         }
       }
-    } else {
-      updateImaStateForPlayerState();
     }
+    updateImaStateForPlayerState();
   }
 
   // Internal methods.