Keep empty strings

[cdfre]

If i change a translation to an empty string, the value is overwritten.

example 1:

my:
    example:
        key: ''

Without @desc / | desc in code / twig and the option "--keep", the result is:

my:
    example:
        key: my.example.key

example 2:

my:
    example:
        # Desc: Mein Beispiel Schlüssel
        key: ''

The result is:

my:
    example:
        # Desc: Mein Beispiel Schlüssel
        key: 'Mein Beispiel Schlüssel'

[schmittjoh]

Fixed by #43