Merge two sorted linked lists and return it as a sorted list. The list should be made by splicing together the nodes of the first two lists.  Example 1: Input: l1 = [1,2,4], l2 = [1,3,4] Output: [1,1,2,3,4,4]  Example 2: Input: l1 = [], l2 = [] Output: []  Example 3: Input: l1 = [], l2 = [0] Output: [0]  Constraints: The number of nodes in both lists is in the range [0, 50].