Allow empty commands as a way to override actions.
Allow empty commands as a way to override actions.

Users can now override actions with non-actions by providing empty strings
instead of real commands in the configuration. Those empty strings will be
selected by the filtering code, but won't be executed.

fernandotcl/udisks-glue,fernandotcl/udisks-glue,fernandotcl/udisks-glue