fix: default merged resolver behavior for nullable fields

[carmelid]

TODO:

• If this PR is a new feature, reference an issue where a consensus about the design was reached (not necessary for small changes)
• Make sure all of the significant new logic is covered by tests
• Rebase your changes on master so that they can be merged easily
• Make sure all tests and linter rules pass
• Update CHANGELOG.md with your change. Include a description of your change, link to PR (always) and issue (if applicable). Add your CHANGELOG entry under vNEXT. Do not create a new version number for your change yourself.

Closes #1171

Even though the contribution guidelines clearly states that there should be tests covering these sorts of bug fixes, I have not been able to write a test that reproduces the issue. Help and suggestions are very much appreciated (If we conclude this fix needs tests that is)

[apollo-cla]

@carmelid: Thank you for submitting a pull request! Before we can merge it, you'll need to sign the Meteor Contributor Agreement here: https://contribute.meteor.com/

[carmelid]

Ping @abernix @trevorblades @hwillson could anyone of you please take a look at this fix :)

[yaacovCR]

Closed in favor of implementation in #1307

Thanks for pointing this out!