Reset to null a nullable field

[Dri372]

Is your feature request related to a problem? Yes
I'm always frustrated when a enter a value in a nullable field and than I cannot edit it and set it back to Null.

Describe the solution you'd like
Input Null to set to Null a field
For text field, the text Null should be enter with "Null"

Describe alternatives you've considered
Direct sql in the db