Fixed there can't be submitted settings

I can't be submitted when there has null in the setting.

TODO:

• Keep ''.
  When using S3 and CDN, it is necessary to make truncated_namespace empty, but it is necessary to correct it because it has erased the empty character.

changed the description

changed the description

mentioned in commit c9684059

merged

mentioned in commit 90860b08

mentioned in issue #34 (closed)