Description: the issue only occurs in ee-6.2.x. On master, we used different way to do this and its value is empty on ddmcontent table.
1. Create a Data Definition named "Test DD" add a 'Number' field to it.
2. In the Dynamic Data List, create a list "Test List" using "Test DD" as the definition.
3. Add record under "Test List" and leave the field number empty. -> publish.
Actual Behavior: check xml field in ddmcontent table, its number field value is null.
Expected Behavior: check xml field in ddmcontent table, its number field value should empty.