docs: clarify what "empty" is

Author: kenjis

user_guide_src/source/models/model.rst

@@ -180,13 +180,13 @@ $createdField
 ^^^^^^^^^^^^^
 
 Specifies which database field to use for data record create timestamp.
-Leave it empty to avoid updating it (even if ``$useTimestamps`` is enabled).
+Leave it empty (``''``) to avoid updating it (even if ``$useTimestamps`` is enabled).
 
 $updatedField
 ^^^^^^^^^^^^^
 
 Specifies which database field should use for keep data record update timestamp.
-Leave it empty to avoid update it (even ``$useTimestamps`` is enabled).
+Leave it empty (``''``) to avoid update it (even ``$useTimestamps`` is enabled).
 
 $deletedField
 ^^^^^^^^^^^^^