soundwave: Gracefully handle empty timeseries response.

BUG=986679

Change-Id: I60a17ce118f64291d9bc263ad587c87a651b075a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1764560Reviewed-by: Juan Antonio Navarro Pérez <perezju@chromium.org>
Commit-Queue: Sadrul Chowdhury <sadrul@chromium.org>
Cr-Commit-Position: refs/heads/master@{#690797}

tools/perf/cli_tools/soundwave/tables/timeseries.py

@@ -129,6 +129,10 @@ def _DataFrameFromJsonV2(ts_key, data):
 
 
 def _DataFrameFromJsonV1(test_path, data):
+  # The dashboard API returns an empty list if there is no recent data for the
+  # timeseries.
+  if not data:
+    return DataFrame()
   assert test_path == data['test_path']
   config = _ParseConfigFromTestPath(data['test_path'])
   config['improvement_direction'] = _CODE_TO_IMPROVEMENT_DIRECTION.get(