extra/runner/index.html: Display the plot with no passed test

jqplot has a limitation that it considers the data set passed to its
constructor invalid if plotSeries[0] is an empty array, which is the
case when no completed tests passed.

Work around this limitation by passing a dummy element to plotSeries[0]
if it's otherwise empty. This solution has a drawback that it prints a
fake point to the scatter plot, but it's hardly visible because its value
is 0 and it ensures that the rest of the graph is displayed correctly.