Adding default key to the source and key configuration values. No default value lowers the charm proof warning to information.

config.yaml

@@ -119,6 +119,7 @@ options:
         a hook error.
   source:
       type: string
+      default:
       description: |
         Optional configuration to support use of additional sources such as:
         .
@@ -130,6 +131,7 @@ options:
         option.
   key:
       type: string
+      default:
       description: |
         Key ID to import to the apt keyring to support use with arbitary source
         configuration from outside of Launchpad archives or PPA's.