fix: remove variable conflicts by making instance_type nullable

[Woodstein-Inc]

Description

Made instance_type nullable in the self-managed-node-groups module

Motivation and Context

#3453

Breaking Changes

N/A

How Has This Been Tested?

• I have updated at least one of the examples/* to demonstrate and validate my change(s)
• I have tested and validated these changes using one or more of the provided examples/* projects

• I have executed pre-commit run -a on my pull request

When making the change in this PR the following code works.

<...>
      instance_type = null
      instance_requirements = {
        vcpu_count = {
          min = 2
        }
        memory_mib = {
          min = 8192
        }
        instance_generations = ["current", "previous"]
      }
<...>

[antonbabenko]

This issue has been resolved in version 21.0.6 🎉

[github-actions]

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