fix(formValidator): change string concatenation to "null-coalesce"
fix(formValidator): change string concatenation to "null-coalesce"

when there is no message set to a specific type of error the validator will set undfined as a string to the message so only add to the message if there is a value. 
fix #45
calvingerling/booktionator,calvingerling/booktionator,calvingerling/booktionator