null performance handling

[johnharding]

bug - if performance value is null, application crashes

[johnharding]

This is trickier than it first appears. At present, there is kind of a solution whereby nulls are replaced with zeros, however this is not great if your goal is to minimise a performance criteria.

Something needs to flag when a performance value is a certain predefined number (not null), and then discount it.

[johnharding]

Need to make sure this is covered for designs that are not cluster representatives