Changed default values for min and max to null

The previous defaults 0 and false were converted to the integers 0 and 0, resulting in 0 being the only valid value for the given integer field.

Fixes EZP-21108, EZP-21109.
Merge pull request #844 from jeromegamez/EZP-21108-21109_ezinteger

EZP-21109: Changed default values for min and max to null