Support unmapped adjacency list indices

[HusseinElMotayam]

There is a slight improvement that can be done based on the discussion in:
tomshanley/d3-sankey-circular#40

Suppose that the adjacency list is defined as:

var adjacencyList = [];
adjacencyList[0] = [2];
adjacencyList[2] = [0];

Iterating sequentially over that array in https://github.com/antoinerg/elementary-circuits-directed-graph/blob/master/johnson.js#L72 will throw an error when reaching index 1.

Perhaps we can do a check first and skip non-existing keys.
I'm also not sure if this will be sufficient, or if other changes need to be done somewhere else to accommodate the missing keys in the adjacency list.