Tweak null-checking in TextRenderer#getNextEventTime()

`subtitle` is only guaranteed to be non-null if
`nextSubtitleEventIndex != C.INDEX_UNSET`. The null check added in
https://github.com/google/ExoPlayer/commit/0efec5f6c12a5d583f24c122fbcbc1b1eebbabc3
was too early.

Issue: #8017
PiperOrigin-RevId: 334777742

RELEASENOTES.md

@@ -19,6 +19,8 @@
     *   Add support for `\h` SSA/ASS style override code (non-breaking space).
     *   Fix WebVTT subtitles in MP4 containers in DASH streams
         ([#7985](https://github.com/google/ExoPlayer/issues/7985)).
+    *   Fix NPE in `TextRenderer` when playing content with a single subtitle
+        buffer ([#8017](https://github.com/google/ExoPlayer/issues/8017)).
 *   UI:
     *   Do not require subtitleButton in custom layouts of StyledPlayerView
         ([#7962](https://github.com/google/ExoPlayer/issues/7962)).

library/core/src/main/java/com/google/android/exoplayer2/text/TextRenderer.java

@@ -325,10 +325,13 @@ public final class TextRenderer extends BaseRenderer implements Callback {
   }
 
   private long getNextEventTime() {
+    if (nextSubtitleEventIndex == C.INDEX_UNSET) {
+      return Long.MAX_VALUE;
+    }
     checkNotNull(subtitle);
-    return nextSubtitleEventIndex == C.INDEX_UNSET
-        || nextSubtitleEventIndex >= subtitle.getEventTimeCount()
-        ? Long.MAX_VALUE : subtitle.getEventTime(nextSubtitleEventIndex);
+    return nextSubtitleEventIndex >= subtitle.getEventTimeCount()
+        ? Long.MAX_VALUE
+        : subtitle.getEventTime(nextSubtitleEventIndex);
   }
 
   private void updateOutput(List<Cue> cues) {