runner_on_start events have zero-length strings for their stdout
fields. We don't want to display these in the ui so we omit them.
Although the stdout field is an empty string, it still has a recorded
line length of 1 that we must account for. Since we're not rendering
the blank line, we must also go back and set the event record's line
length to 0 in order to avoid deleting too many lines when we pop or
shift events off of the view while scrolling.