Hole type: subroutine
Write a subroutine that accepts a string which may consist of a
repeating pattern, and returns the smallest repeating substring. If
the string does not consist of a repeating pattern, the subroutine
should return undef or the empty string.
input output 'aaaaaa' 'a' 'ababab' 'ab' 'aabaab' 'aab' 'ababaa' ''