Use identity check with PLACEHOLDER instead of equality test

[atomicmac]

Summary

PLACEHOLDER is a specific instance of a sentinel object, the test here should be identity instead of equality, i.e. "is not" instead of "!=" I am experimenting with adding ndarray support, and the equality test here causes problems.

Checklist

• [x] If code changes were made then they have been tested.
  • [ ] I have updated the documentation to reflect the changes.
• [x] This PR fixes an issue.
• [ ] This PR adds something new (e.g. new method or parameters).
  • [ ] This change has an associated test.
• [ ] This PR is a breaking change (e.g. methods or parameters removed/renamed)
• [ ] This PR is not a code change (e.g. documentation, README, ...)