Fix divide-by-zero crash in animation_bench benchmark (#137539)

If the benchmark runs out of time before it closes the drawer it is animating, it tries to divide by zero when computing the time per frame.

Don't report time per frame for activities with zero frames. This likely only happens for the close frame action, but guards are added to all time per frame computations in this benchmark.

dev/benchmarks/microbenchmarks/lib/stocks/animation_bench.dart

@@ -83,23 +83,29 @@ Future<void> main() async {
     unit: 's',
     name: 'stock_animation_total_run_time',
   );
+  if (totalOpenIterationCount > 0) {
     printer.addResult(
       description: '  Opening first frame average time',
       value: totalOpenFrameElapsedMicroseconds / totalOpenIterationCount,
       unit: 'µs per frame ($totalOpenIterationCount frames)',
       name: 'stock_animation_open_first_frame_average',
     );
+  }
+  if (totalCloseIterationCount > 0) {
     printer.addResult(
       description: '  Closing first frame average time',
       value: totalCloseFrameElapsedMicroseconds / totalCloseIterationCount,
       unit: 'µs per frame ($totalCloseIterationCount frames)',
       name: 'stock_animation_close_first_frame_average',
     );
+  }
+  if (totalSubsequentFramesIterationCount > 0) {
     printer.addResult(
       description: '  Subsequent frames average time',
       value: totalSubsequentFramesElapsedMicroseconds / totalSubsequentFramesIterationCount,
       unit: 'µs per frame ($totalSubsequentFramesIterationCount frames)',
       name: 'stock_animation_subsequent_frame_average',
     );
+  }
   printer.printToStdout();
 }