Do not apply COALESCE on required Char

Closes #14909
(grafted from 776fe202ed6fd03b690948cf4e2dabc2fd28bbc4)

--HG--
branch : 8.0

trytond/trytond/model/fields/char.py

@@ -114,7 +114,8 @@ class Char(FieldTranslate):
 
     def _domain_column(self, operator, column):
         column = super()._domain_column(operator, column)
-        column = Coalesce(column, '')
+        if not self.required:
+            column = Coalesce(column, '')
         if self.search_unaccented and operator.endswith('ilike'):
             database = Transaction().database
             column = database.unaccent(column)

trytond/trytond/model/modelsql.py

@@ -349,7 +349,9 @@ class ModelSQL(ModelStorage):
             rec_name_field = getattr(cls, cls._rec_name, None)
             if (isinstance(rec_name_field, fields.Field)
                     and not hasattr(rec_name_field, 'set')):
-                column = Coalesce(Column(table, cls._rec_name), '')
+                column = Column(table, cls._rec_name)
+                if not rec_name_field.required:
+                    column = Coalesce(column, '')
                 if getattr(rec_name_field, 'search_unaccented', False):
                     column = Index.Unaccent(column)
                 cls._sql_indexes.add(