check_graphite improvements

--warning is optional. only required if you want less-than comparisons.
--no-data-as-zero renamed to -null-data-as-zero for clarity.

null data points should count when figuring the average value.
previously, a series [null, null, 5, null, null] would have an average of 5,
since only non-null data points were included in the divisor when calculating
the mean. now, the average will be 1.

this is different than --null-data-as-zero. that flag matters most when
ALL data points are null. without this flag, a series [null, null, null]
raises a 'no data' critical error. with this flag, the series becomes
[0, 0, 0], so an average of 0. This may or may not be an error depending on the
warn/crit thresholds.