Do not convert if the localized string is empty

[thii]

Instead of putting an empty string, leaving it out and iOS will automaticallly
choose the closest available localizable string.

[houndci-bot]

Trailing whitespace detected.

[netbe]

@thii thanks for the contribution, could you explain more about your PR, I am not sure to understand the gain here?

And then to be accepted:

1. Write a test case
2. Make sure the all the tests pass: right now this feature breaks following test:

Failure: test_converting_csv_to_dotstrings_with_no_default_lang_is_empty(TestCSV2Strings)
/Users/netbe/Code/Babelish/test/babelish/test_csv2strings.rb:53:in `test_converting_csv_to_dotstrings_with_no_default_lang_is_empty'
     50:     converter.convert
     51:     assert File.exist?(spanish_file), "the ouptut file does not exist"
     52:     result = File.read(spanish_file)
  => 53:     assert_equal expected_output, result
     54:     system("rm -rf *.lproj")
     55:   end
     56:
<"\"GREETINGS\"      = \"Buenos dias\";\n\"ANOTHER_STRING\" = \"\";\n"> expected but was
<"\"GREETINGS\"      = \"Buenos dias\";\n">

diff:
  "GREETINGS"      = "Buenos dias";
- "ANOTHER_STRING" = "";

1. Could you fix the hound violations ?

Thank you

[coveralls]

Changes Unknown when pulling 47d451a on thii:empty-string into * on netbe:master*.

[netbe]

please rebase with master so tests can run on travis