Fix CI linter error when repository is empty
What does this MR do and why?
NoMethodError: undefined method `sha' for nil:NilClass
@root_ref_sha ||= head_commit.sha
^^^^
from app/models/repository.rb:982:in `root_ref_sha'
from lib/api/lint.rb:28:in `block (2 levels) in <class:Lint>'
from grape (2.0.0) lib/grape/endpoint.rb:58:in `call'
Edited by Grzegorz Bizon