Correctly handle null values when executed with a callback

[sstone1]

The following error occurs when jsonata is invoked with a callback and an expression that returns a null value:

TypeError: Cannot read property 'then' of null
      at evaluate (jsonata.js:1377:58)
      at next (native)
      at evaluateStep (jsonata.js:1476:31)
      at next (native)
      at evaluatePath (jsonata.js:1429:38)
      at next (native)
      at evaluate (jsonata.js:1323:34)
      at next (native)
      at Object.evaluate (jsonata.js:4117:33)
      at test/async-function.js:20:14
      at jsonataPromise (test/async-function.js:19:12)
      at Context.<anonymous> (test/async-function.js:95:23)

[coveralls]

Coverage remained the same at 100.0% when pulling 7ae8782 on sstone1:async-error-on-null into 502c2e3 on jsonata-js:master.

[andrew-coleman]

Thank you!