change non_empty? method in lib/regexp_m17n.rb to make test pass

If its compatible, then you don't need to do anything.

```ruby
regex = Regexp.new(pattern)
```

should be enough.

or

```ruby
regex =
  if Encoding.compatible?(str, pattern)
    Regexp.new(pattern)
  else
    Regexp.new(pattern.encode(str.encoding), Regexp::FIXEDENCODING)
  end
```