There are three ways in which null values can be handled:

1 . Zero. When this option is used, each null value is replaced with a zero. The data entry is not removed from the data collection. Only its original null/empty value is replaced with zero. 
2 . Gap. This option visually removes the regions for each set of empty(null) points. The data entry is not removed from the data collection. Essentially, it connects the points neighboring the null point (or the collection of empty values).
3 . Drop - this option clips, or visually removes each section/segment, corresponding to a set of empty points/values.