[24.0] Fix `null` values in tool expression

[davelopez]

Fixes #17960

WIP

How to test the changes?

(Select all options that apply)

• [ ] I've included appropriate automated tests.
• [ ] This is a refactoring of components with existing test coverage.
• [x] Instructions for manual testing are as follows:
  • Create a paste upload with null and set the datatype to expression.json
  • Create a Workflow and add a step with Expression Tools -> Parse parameter value
  • Run the workflow passing the expression.json dataset as input
  • Observe the result

License

• [x] I agree to license these and all my past contributions to the core galaxy codebase under the MIT license.