Differentiate scalar input artifacts against single item lists

[schustmi]

Describe changes

Fixes a bug where a list of input artifacts of length 1 gets treated as a scalar input artifact and fails because of materializer type incompatibility.

Closes #4647.

Pre-requisites

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• I have read the CONTRIBUTING.md document.
• I have added tests to cover my changes.
• I have based my new branch on develop and the open PR is targeting develop. If your branch wasn't based on develop read Contribution guide on rebasing branch to develop.
• IMPORTANT: I made sure that my changes are reflected properly in the following resources:
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  • Dashboard: Needs to be communicated to the frontend team.
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Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Other (add details above)