key=23;0;0;0;0 and key=23;;;; lead to division by zero exceptions
This should fix this by ignoring empty strings for min/max (formerly
it got "converted" to float(0)) and also ignores min == max.
fixes#6828
Prior to this change the PerfdataSet only knew the labels of a performance
data value which prevented the Perfdata object from being used
individually.
refs #6515
Thomas Gelf
Johannes Meyer
Matthias Jentsch