Support unnamed restarg

[TastyPi]

Without this change, the test produces the following diagnostics:

diagnostics:
- range:
    start:
      line: 2
      character: 10
    end:
      line: 2
      character: 11
  severity: ERROR
  message: Cannot detect the type of the expression
  code: Ruby::FallbackAny
- range:
    start:
      line: 2
      character: 13
    end:
      line: 2
      character: 15
  severity: ERROR
  message: Cannot detect the type of the expression
  code: Ruby::FallbackAny
- range:
    start:
      line: 3
      character: 11
    end:
      line: 3
      character: 12
  severity: ERROR
  message: Cannot detect the type of the expression
  code: Ruby::FallbackAny