OPT: check element fisrt

[HOUCe]

run if (!element) at first to avoid rest code running.

If if (!element) equals true, then stop executing the rest other wise we run element.forEach(item => walkTree(item, context, visitor, newContext)); all the time at first

Checklist:

• If this PR is a new feature, please 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
• If this was a change that affects the external API used in GitHunt-React, update GitHunt-React and post a link to the PR in the discussion.

[apollo-cla]

@HOUCe: 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/

[hwillson]

Great idea - thanks @HOUCe!