refactor: replaces lodash' flattenDeep, isEmpty, reject, omit, merge with native alternatives

[sverweij]

Description

• replaces lodash' flattenDeep, isEmpty, reject, omit, merge with native alternatives

Motivation and Context

Part of a series of PR's to phase out lodash (which is not maintained anymore) as a production dependency.

How Has This Been Tested?

• green ci
• additional unit test

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• Documentation only change
• Refactor (non-breaking change which fixes an issue without changing functionality)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist

• 📖

  • My change doesn't require a documentation update, or ...
  • it does and I have updated it

• ⚖️

  • The contribution will be subject to The MIT license, and I'm OK with that.
  • The contribution is my own original work.
  • I am ok with the stuff in CONTRIBUTING.md.

[codeclimate]

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