use null when importing empty string on a field with required=False

It looks like it is what the odoo import does, without that, there is no way to differentiate null and the empty string

odoo_scripts/import_jsonrpc.py

@@ -50,7 +50,7 @@
             )
         return xmlid_cache[xmlid]
 
-    def _get_ttype(model: str, field_name: str) -> str:
+    def _get_imf(model: str, field_name: str):
         imf_ids = imf.search(
             [("model", "=", model), ("name", "=", field_name)]
         )
@@ -54,8 +54,8 @@
         imf_ids = imf.search(
             [("model", "=", model), ("name", "=", field_name)]
         )
-        return imf.browse(imf_ids)[0].ttype
+        return imf.browse(imf_ids)[0]
 
     def _convert_identity(value):
         return value
 
@@ -58,7 +58,10 @@
 
     def _convert_identity(value):
         return value
 
+    def _convert_identity_nullable(value):
+        return value if value else None
+
     def _convert_bool(value) -> bool:
         return value in (1, "1", "True", "true", True)
 
@@ -68,7 +71,7 @@
     def _convert_float(value) -> float:
         return float(value)
 
-    def _convert_many2one(xmlid: str) -> int:
+    def _convert_many2one(xmlid: str) -> Union[None, int]:
         if xmlid:
             return _ref(xmlid)
         return None
@@ -106,8 +109,9 @@
                         ):
                             odoo_field_name = column_name[:-3]
                         if odoo_field_name != "id":
-                            ttype = _get_ttype(model, odoo_field_name)
-                            _logger.debug(
+                            field = _get_imf(model, odoo_field_name)
+                            ttype = field.ttype
+                            _logger.info(
                                 "%s.%s type is %s",
                                 model,
                                 odoo_field_name,
@@ -121,7 +125,10 @@
                                 "selection",
                                 "date",
                             ):
-                                converter = _convert_identity
+                                if field.required:
+                                    converter = _convert_identity
+                                else:
+                                    converter = _convert_identity_nullable
                             elif ttype in ("many2one",):
                                 converter = _convert_many2one
                             elif ttype in ("many2many",):