delivery_method can be null

[ikudosi]

Docs does not say it, but after playing around with the fulfillment orders API, there were some instances where it came back as null.

[coveralls]

Coverage: 100.0%. Remained the same when pulling 0e04a7c95aad4214cdcdc530012096fdd2d31e9b on ikudosi:patch-3 into 73007be958a7bbc5435631c18123b482b3b0c9c8 on MONEI:master.

[lpinca]

Thank you.