Write a function, linked_list_values, that takes in the head of a linked list as an argument. The function should return a list containing all values of the nodes in the linked list.
a = Node("a")
b = Node("b")
c = Node("c")
d = Node("d")
a.next = b
b.next = c
c.next = d
a -> b -> c -> d
linked_list_values(a) # -> [ 'a', 'b', 'c', 'd' ]
x = Node("x")
y = Node("y")
x.next = y
# x -> y
linked_list_values(x) # -> [ 'x', 'y' ]
q = Node("q")
# q
linked_list_values(q) # -> [ 'q' ]
linked_list_values(None) # -> [ ]