Fix saving native AMP scaler state

[kepler]

Before submitting

• Was this discussed/approved via a Github issue? (no need for typos and docs improvements)
• Did you read the contributor guideline, Pull Request section?
• Did you make sure to update the docs?
• Did you write any new necessary tests?
• If you made a notable change (that affects users), did you update the CHANGELOG?

What does this PR do?

Fixes saving native AMP scaler state that was introduced in #1561.

PR review

Anyone in the community is free to review the PR once the tests have passed.

[codecov]

Codecov Report

Merging #1777 into master will not change coverage.
The diff coverage is 100%.

@@          Coverage Diff           @@
##           master   #1777   +/-   ##
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  Coverage      88%     88%           
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  Files          69      69           
  Lines        4302    4302           
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  Hits         3793    3793           
  Misses        509     509