Error in Bootstrapping shield from fresh database

[krtkvrm]

Describe the bug
If starting from fresh postgres, we aren't able to bootstrap the DB and SpiceDB Schema, we get this error:

Cause
We aren't passing any NamespaceID while creating any role, thus we are getting.
In #131, this line was removed which was check for namespace_id to be empty or not - internal/store/postgres/role.go#L130

PG Error:

• 

Debug:

• 

To Reproduce
Steps to reproduce the behavior:

1. Start a new PG Instance and run shield migrate
2. Try to run shield serve

Expected behavior
We should be successfully bootstrapping the roles, namespaces, actions and schema

[mabdh]

This could also relate to #136