exometer_admin:default/1 prevents creation of custom types

[uwiger]

exometer_admin:default(Type) will cause a function_clause exception for any given type not already known to exometer_core. This seems unnecessary.

Fix: have a catch-all clause return undefined. If no module is given for the type, this will cause a follow-on error, which perhaps could be handled in a prettier way.

(Would create a PR directly, but am already messing with exometer_admin for other reasons, so this is a reminder for later.)