I guess you want the field to become read only once the end user has enteded some Content? In that case the field should be read write in the New form and read only in the Edit form which can be done by inserting a tiny script in the Edit form
Found it... there seems to be an additional property used by SharePoint scripts that enables and disables the field.
To make the field editable again use the following line
This functionality works only in forms not on Quick Edit Views like most JS validation solutions