fix: Prevent crash in GetEarliestTimestamp() if periods are empty (#1173)

While I have not yet found why the periods are empty, this will prevent shaka from seg faulting

Fixes #1172

packager/mpd/base/mpd_builder.cc

@@ -342,6 +342,8 @@ float MpdBuilder::GetStaticMpdDuration() {
 bool MpdBuilder::GetEarliestTimestamp(double* timestamp_seconds) {
   DCHECK(timestamp_seconds);
   DCHECK(!periods_.empty());
+  if (periods_.empty())
+    return false;
   double timestamp = 0;
   double earliest_timestamp = -1;
   // TODO(kqyang): This is used to set availabilityStartTime. We may consider