NIFI-8266: Fix NPE when validating supported properties against validation context

[mattyb149]

Thank you for submitting a contribution to Apache NiFi.

Please provide a short description of the PR here:

Description of PR

Rather than getting the component's supported properties and validating them using a context that may or may not have the same properties, this change just iterates over the context's properties and validates those. 99% of the time the properties match, but for scripted components that can add properties once the script is loaded, it was causing a NullPointerException.

In order to streamline the review of the contribution we ask you
to ensure the following steps have been taken:

For all changes:

• Is there a JIRA ticket associated with this PR? Is it referenced
  in the commit message?

• Does your PR title start with NIFI-XXXX where XXXX is the JIRA number you are trying to resolve? Pay particular attention to the hyphen "-" character.

• Has your PR been rebased against the latest commit within the target branch (typically main)?

• Is your initial contribution a single, squashed commit? Additional commits in response to PR reviewer feedback should be made on this branch and pushed to allow change tracking. Do not squash or use --force when pushing to allow for clean monitoring of changes.

For code changes:

• Have you ensured that the full suite of tests is executed via mvn -Pcontrib-check clean install at the root nifi folder?
• Have you written or updated unit tests to verify your changes?
• Have you verified that the full build is successful on JDK 8?
• Have you verified that the full build is successful on JDK 11?
• If adding new dependencies to the code, are these dependencies licensed in a way that is compatible for inclusion under ASF 2.0?
• If applicable, have you updated the LICENSE file, including the main LICENSE file under nifi-assembly?
• If applicable, have you updated the NOTICE file, including the main NOTICE file found under nifi-assembly?
• If adding new Properties, have you added .displayName in addition to .name (programmatic access) for each of the new properties?

For documentation related changes:

• Have you ensured that format looks appropriate for the output in which it is rendered?

Note:

Please ensure that once the PR is submitted, you check GitHub Actions CI for build issues and submit an update to your PR as soon as possible.

[markap14]

Thanks @mattyb149 changes look good to me. Kept minimal and improvement makes a lot of sense. Was able to create a flow using FreeFormTextWriter, rebuild with the new PR, and still bring up the flow, so we know that it doesn't break any existing flows. All tests pass, and all system tests still pass. +1 merged to main.