Update converter._to_field and to_model to fix a bug.

modules/plugin_dbui/converter.py

@@ -144,7 +144,7 @@ def _to_field(field, linkedcombo=True, **kwargs):
     # in addition the JSON dumps will failed
     # therefore the default value are limited to types which can be JSONify.
     default = field.default
-    if default and isinstance(default, JSON_TYPES):
+    if default is not None and isinstance(default, JSON_TYPES):
         cfg["value"] = default
 
         # the checkBox is rendered initially checked
@@ -806,7 +806,7 @@ def to_model(table):
         # therefore the default value are limited to types which
         # can be JSONifyied.
         default = field.default
-        if default and isinstance(default, JSON_TYPES):
+        if default is not None and isinstance(default, JSON_TYPES):
             field_cfg['defaultValue'] = default
 
         cfg['fields'].append(field_cfg)