5.0.3 haml -c --stdin generates an exception when checking ruby code

[moorecp]

It looks like commit 83da985 caused an issue when using --stdin instead of specifying a filename. If there are no errors in your HAML, when validate_ruby is called, @options[:filename] is nil and an exception is generated. For example:

# haml -c --stdin
%foo
  %bar

This results in

Exception on line 341: no implicit conversion of nil into String
  Use --trace for backtrace.

Previously, this would result in

Syntax OK

This causes issues with on the fly syntax checkers that utilize STDIN to validate without saving.

[k0kubun]

resolved by #958