Allow to fallback to key when trans string is empty

[limdauto]

I think it's sensible to display the key (sometimes people use English string or whichever default language they use as key) instead of an empty string.

[captolia]

+1

[timseverien]

+1

[jamuhl]

if i accept this PR the other 50% will complain that they want empty string as an acceptable value (was like this before).

make that depending on a setting provide on init showKeyIfEmpty (like fallbackOnEmpty, fallbackOnNull)

[limdauto]

@jamuhl cool. I will add that init option. Thanks for replying.

[limdauto]

@jamuhl sorry for the delay. I have added the init option. Could you please take a look? Thnx

[jamuhl]

perfectm, thank you...will merge this asap.