fix set_backtrace signature when passing nil value

[HoneyryderChuck]

irb(main):001:0> e = StandardError.new
=> #<StandardError: StandardError>
irb(main):002:0> e.set_backtrace nil
=> nil
irb(main):003:0> e.set_backtrace "bang"
=> ["bang"]

without this change, there are steep errors everywhere where the following idiommatic ruby expression is used:

def raise_another(error)
  ex =  StandardError.new
  ex.set_backtrace(error.backtrace)
  raise ex
end

because error.backtrace can be nil.

[soutaro]

Thank you!