Fix staticcheck nil pointer warnings in registration_test.go

servers/runner/registration_test.go has three SA5011 (possible nil pointer dereference) staticcheck warnings at lines 78, 566, and 595. These are on main and cause the lint job to fail on every PR.

Example pattern — a nil check suggests the pointer can be nil, but it's dereferenced unconditionally a few lines later:

• Line 75 (nil check) → line 78 (dereference)
• Line 563 (nil check) → line 566 (dereference)
• Line 592 (nil check) → line 595 (dereference)

Spotted while CI ran on #850.