When a metric has a null value, number formatters like
`humanizeNoSmallPrefix` will throw "Uncaught TypeError: Cannot read
property 'toPrecision' of null".

This is fixed by explicitly checking for `null` and returning the string
"null".

Note: This is usually not seen as rickshaw doesn't show annotations for
null values, but still calls the formatter.

Signed-off-by: David Coles <coles.david@gmail.com>