getUserToBlamePk() returns null due to referencing .env

[zenoandras]

In certain cases the .env config is not available for the blame to fall back on when calling getUserToBlamePk().
This results in a null result which immediately triggers an unhandled (and uncatchable) error exception as the return value has to be :string.

The solution would be to publish the system_user_id into the public config with a null default value and make it available for project-specific override (to avoid null calls).
It is a preferred Laravel pattern to use config() instead of env() anyways.

[zenoandras]

PR: #19