On Maria DB, null fields are updated due to bad management of default value

[openlowcode]

Since 1.6, there is now management of field definition, with possibility to update fields
( #80 ). It seems there is an error when:

• a string field has by default NULL value (appears as NULL text in the data-model query)
• the field is uselessly reinitialised.

[openlowcode]

org.openlowcode.server.data.storage.standardjdbc.StandardJDBCTableFieldCheck.visitIncompatible default value for field ID, default value in db = NULL, in model = null
org.openlowcode.server.data.DataObjectDefinition.updatePersistenceStorage PERSISTENCE: modifying field ID in table BM_ALLOCATEDBUDGET