Fix bugs in schema code

[vyasr]

Description

This PR fixes some minor issues I found while implementing glotzerlab/signac-flow#504.

Motivation and Context

These bug fixes have no impact right now because migrations are not enabled, but they will be needed once the migration code path is activated.

Types of Changes

• Documentation update
• Bug fix
• New feature
• Breaking change¹

¹The change breaks (or has the potential to break) existing functionality.

Checklist:

• I am familiar with the Contributing Guidelines.
• I agree with the terms of the Contributor Agreement.
• My name is on the list of contributors.
• My code follows the code style guideline of this project.
• The changes introduced by this pull request are covered by existing or newly introduced tests.

If necessary:

• I have updated the API documentation as part of the package doc-strings.
• I have created a separate pull request to update the framework documentation on signac-docs and linked it here.
• I have updated the changelog and added all related issue and pull request numbers for future reference (if applicable). See example below.

I don't think a changelog entry is warranted for this PR.

[bdice]

Looks good to me.

[codecov]

Codecov Report

Merging #540 (5784952) into master (ff208cf) will not change coverage.
The diff coverage is 50.00%.

❗ Current head 5784952 differs from pull request most recent head fccb4cf. Consider uploading reports for the commit fccb4cf to get more accurate results

@@           Coverage Diff           @@
##           master     #540   +/-   ##
=======================================
  Coverage   78.27%   78.27%           
=======================================
  Files          65       65           
  Lines        7069     7069           
  Branches     1311     1311           
=======================================
  Hits         5533     5533           
  Misses       1231     1231           
  Partials      305      305           

Impacted Files	Coverage Δ
signac/__main__.py	71.89% <ø> (ø)
signac/contrib/migration/__init__.py	82.45% <50.00%> (ø)

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