JSON output doesn't cope with endpoints present in some but not all earlier builds

[hgcummings]

In the case where an endpoint fails (so isn't included in the output) then reappears, the new value is appended to the list of values without any null value in between to represent the skipped build.

Note that the JSON reporter should possibly output all URLs rather than just failing ones, which would mitigate this (but it would still need solving anyway).