Complete the function that takes an array of words.
You must concatenate the nth letter from each word to construct a new word which should be returned as a string, where n is the position
of the word in the list.
For example:
["yoda", "best", "has"] --> "yes"
^ ^ ^
n=0 n=1 n=2
Note: Test cases contain valid input only - i.e. a string array or an empty array; and each word will have enough letters.