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

• If code changes were made then they have been tested.
  • I have updated the documentation to reflect the changes.
• 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, ...)

[Gobot1234]

FWIW ndarray support has been rejected recently in favour of a rust based deserialiser. See #517