Use `null` CI input value to remove parent node

What does this MR do and why?

This commit implements the ability to pass null to a CI input of any type. If that input is used as the only value of a key, the key will be removed from the config. Users have requested this feature so they can write components in a way that allows consumers of the component to decide whether they want a job in the component to have needs, rules, parallel, etc.

Changelog: added

Issue: #440468

How to set up and validate locally

1. In any project, create a file included.yml with this content:

   spec:
     inputs:
       build_needs:
         type: array
         default: null
       test_needs:
         type: array
       deploy_needs:
         type: array
   
   ---
   
   build:
     script: echo "$[[ inputs.build_needs ]]"
     needs: $[[ inputs.build_needs ]]
   
   test:
     script: echo "$[[ inputs.test_needs ]]"
     needs: $[[ inputs.test_needs ]]
   
   deploy:
     script: echo "$[[ inputs.deploy_needs ]]"
     needs: $[[ inputs.deploy_needs ]]

2. Paste this configuration into the project's CI editor:

   include:
     - local: included.yml
       inputs:
         test_needs: null
         deploy_needs: ['build', 'test']

3. See that you get this merged YAML:

   build:
     script: echo ""
   test:
     script: echo ""
   deploy:
     script: echo "["build", "test"]"
     needs:
      - build
      - test