Fix issue when MongoTransform converts object-type field with null values

[yuzeh]

I've been getting gnarly errors when issuing the following request in cURL (to a server running parse-server 2.2.12):

➜ Dan@Dans-MacBook-Pro  ~/MTailor/workspace/parse-server git:(MongoTransform-obj-with-null-values) curl -X POST \
-H "X-Parse-Application-Id: <REDACTED>" \
-H "X-Parse-REST-API-Key: <REDACTED>" \
-H "X-Parse-Master-Key: <REDACTED>" \
-H "Content-Type: application/json" \
-d '{ "orderFormObject": {"nest1": null, "nest2": {"blah": 1}}}' \
https://<REDACTED>/parse/classes/OrderItem
{"code":1,"message":"Internal server error."}%

The trace looks like this:

ESC[31merrorESC[39m: Uncaught internal server error. [TypeError: Cannot convert undefined or null to object] TypeError: Cannot convert undefined or null to object
    at Function.keys (native)
    at transformInteriorValue (/var/app/current/node_modules/parse-server/lib/Adapters/Storage/Mongo/MongoTransform.js:104:100)
    at /var/app/current/node_modules/parse-server/node_modules/lodash/lodash.js:12739:23
    at /var/app/current/node_modules/parse-server/node_modules/lodash/lodash.js:4411:15
    at baseForOwn (/var/app/current/node_modules/parse-server/node_modules/lodash/lodash.js:2654:24)
    at Function.mapValues (/var/app/current/node_modules/parse-server/node_modules/lodash/lodash.js:12738:7)
    at parseObjectKeyValueToMongoObjectKeyValue (/var/app/current/node_modules/parse-server/lib/Adapters/Storage/Mongo/MongoTransform.js:296:28)
    at parseObjectToMongoObjectForCreate (/var/app/current/node_modules/parse-server/lib/Adapters/Storage/Mongo/MongoTransform.js:308:33)
    at MongoStorageAdapter.createObject (/var/app/current/node_modules/parse-server/lib/Adapters/Storage/Mongo/MongoStorageAdapter.js:242:79)
    at /var/app/current/node_modules/parse-server/lib/Controllers/DatabaseController.js:518:29
    at run (/var/app/current/node_modules/parse-server/node_modules/babel-polyfill/node_modules/core-js/modules/es6.promise.js:89:22)
    at /var/app/current/node_modules/parse-server/node_modules/babel-polyfill/node_modules/core-js/modules/es6.promise.js:102:28
    at flush (/var/app/current/node_modules/parse-server/node_modules/babel-polyfill/node_modules/core-js/modules/_microtask.js:18:9)
    at nextTickCallbackWith0Args (node.js:419:9)
    at process._tickDomainCallback (node.js:389:13)

This change should fix the issues I've been having.

[codecov-io]

Current coverage is 91.92%

Merging #2029 into master will not change coverage

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