Return empty string when input is empty instead of 'Invalid Date'

[dehamilton]

I think this should resolve issue #122. Now instead of performing the formatting regardless of 'value' the formatting will only occur if the moment creation is valid. Otherwise it will return an empty string.

[jraeymae]

+1 @chollier : is it possible to merge this one? I need this one urgently...

[wiiiim]

this allows clearing the date, but I suggest a similar fix in the constructor for when you give an empty initial date/time