For typed values other than string, we cannot discern in storage an empty value from a value not yet setted.
For example for a site parameter, how to discern a non initialized parameter (a default value will be proposed) from a site parameter setted to empty (duration of cache validty for example) ?

• Create a new site :
  • Cache duration is proposed to its default value
  • Empty the field and save
• Reconfigure the site
  • The field for cache duration is filled with the default value instead of empty