Fix crashes when source files are empty.

Fixes issue #33.
what if, instead of always checking if `gcov_data` is not `nil`, we just made the `gcov_data` method always return a string. Something like this, in tehre

```ruby
        if File.exists?(gcov_file_name)
          gcov_data = File.new(gcov_file_name).read
        else
          gcov_data = ""
        end
```