Fix raise ArgumentError missing comma in runner fleet seeder

What does this MR do and why

Two guards call a method instead of raising:

raise ArgumentError('runners') unless runners
raise ArgumentError('project_id') unless project_id

ArgumentError(...) is parsed as a method call, and no such method exists, so both guards raise NoMethodError rather than the ArgumentError they name. The seeder is developer tooling, so the cost is a confusing error during seeding rather than a production incident, but the guard still does not do what it says.

How to verify

raise ArgumentError('x')
# => NoMethodError: undefined method `ArgumentError'

Spotted by itaruby(to be released), a Ruby type checker I'm working on.