Join lists
Level: Beginner (score: 2)
Write a function that accepts a list of lists and joins them with a separator character, therefore flattening and separating.
Examples:
>>> join_lists([ ['a', 'b'], ['c'] ], '&')
['a', 'b', '&', 'c']
>>> join_lists([ ['a', 'b'], ['c'], ['d', 'e'] ], '+')
['a', 'b', '+', 'c', '+', 'd', 'e']
Note: Calling the function with an empty list should return None.