Fix exception(backtrace=nil) prints nothing

[tompng]

When an exception that has backtrace=nil is raised, IRB prints nothing.

irb(main):001> raise StandardError.new('error').tap{def _1.backtrace()=nil}
irb(main):002> raise _
irb(main):003> _
=> #<StandardError: error>
irb(main):004> 

I removed if exc.backtrace condition. files?w=1
Any error on printing error with backtrace will be rescued after #780.

The original test properly ensures stderr to be empty, but it was not properly testing stdout.

assert_in_out_err(bundle_exec + %w[-rirb -W0 -e IRB.start(__FILE__) -- -f --], <<-IRB, /Exception: foo/, [])
  e = Exception.new("foo")
  puts e.inspect
  def e.backtrace; nil; end
  raise e
IRB

e = Exception.new("foo")  # prints "=> #<Exception: foo>" that makes test pass
puts e.inspect            # prints "#<Exception: foo>" and "nil" that also makes test pass
def e.backtrace; nil; end # prints "=> :backtrace"
raise e                   # should print some error message and this should be tested but it doesn't