Fixes panic when nonexistent node is queried

[missylbytes]

Changes proposed in this PR:

Adds a nil check for the call back from Consul.

How I've tested this PR:

How I expect reviewers to test this PR:

Checklist:

• Tests added
• CHANGELOG entry added

  Run make changelog-entry for guidance in authoring a changelog entry, and
  commit the resulting file, which should have a name matching your PR number.
  Entries should use imperative present tense (e.g. Add support for...)

[andrewstucki]

Changed a bit of the wording, but should be good to go when we get tests to pass.