Set default value to NULL and authorize column to be NULL for DATE & DATETIME column types

[olivier-nolbert]

This code fixes the error on saving fields on J4.x/S4.1.1 with storage column type set as DATE or DATETIME. It sets default value to NULL instead of 0 and conditionally removes the 'NOT NULL' option in the SQL Query.

[olivier-nolbert]

Error mentioned on SEBLOD forum here : https://www.seblod.com/community/forums/fields-plug-ins/jform-calendar-field-has-more-than-one-problem