Avoid testing truthiness of bokeh source, use len instead
This can be a numpy array in some cases.  We use len to avoid errors like
the following:
ValueError: The truth value of an array with more than one element is
ambiguous. Use a.any() or a.all()
@@ -559,7 +559,7 @@ class TaskStream(components.TaskStream):
if self.index == self.plugin.index:
return
with log_errors():
- if self.index and self.source.data['start']:
+ if self.index and len(self.source.data['start']):
start = min(self.source.data['start'])
duration = max(self.source.data['duration'])
boundary = (self.offset + start - duration) / 1000
