Runtime error when is only debug flags in the code

[Wheatwizard]

If you try to run code with only debug flags and no braces (e.g. #dl) you get the following runtime error:

/Users/eamon/projects/Brain-Flak/stack.rb:51:in `include?': no implicit conversion of nil into String (TypeError)
    from /Users/eamon/projects/Brain-Flak/stack.rb:51:in `is_opening_bracket?'
    from /Users/eamon/projects/Brain-Flak/Interpreter.rb:135:in `step'
    from brain_flak.rb:60:in `<main>'

This error occurs when the interpreter attempts to determine if the program starts with an opening bracket but discovers there is no program. It passes a nil object to the method is_opening_bracket, which then tries to convert that to a string. This error occurs on both the debug-flag-dev and master branches. This error only occurs with debug mode on.

[Wheatwizard]

After commit 434b1a8 a program containing only debug flags no longer throws a runtime exception but rather a brain-flak error. It throws our good friend the Error at character 1: Invalid character ''..