fix(bitswap): 0.4.14 returns empty array instead of null

Since 0.4.14, go-ipfs now returns an empty array rather than null.

This should be considered a breaking change for consumers of this
library.

test/bitswap.spec.js

@@ -33,7 +33,7 @@ describe('.bitswap', function () {
     ipfs.bitswap.wantlist((err, res) => {
       expect(err).to.not.exist()
       expect(res).to.have.to.eql({
-        Keys: null
+        Keys: []
       })
       done()
     })