feat: put string as a placeholder instead of value

[Pompette]

Description

Instead of filling the input with a value that equals to "string", it's now indicated as a placeholder.

Motivation and Context

Fixes #8195

How Has This Been Tested?

Manually testing.
e2e testing.

Screenshots (if appropriate):

My PR contains...

• No code changes (src/ is unmodified: changes to documentation, CI, metadata, etc.)
• Dependency changes (any modification to dependencies in package.json)
• Bug fixes (non-breaking change which fixes an issue)
• [ x] Improvements (misc. changes to existing features)
• Features (non-breaking change which adds functionality)

My changes...

• are breaking changes to a public API (config options, System API, major UI change, etc).
• are breaking changes to a private API (Redux, component props, utility functions, etc.).
• are breaking changes to a developer API (npm script behavior changes, new dev system dependencies, etc).
• [ x] are not breaking changes.

Documentation

• [ x] My changes do not require a change to the project documentation.
• My changes require a change to the project documentation.
• If yes to above: I have updated the documentation accordingly.

Automated tests

• My changes can not or do not need to be tested.
• [ x] My changes can and should be tested by unit and/or integration tests.
• [x ] If yes to above: I have added tests to cover my changes.
• [x ] If yes to above: I have taken care to cover edge cases in my tests.
• [x ] All new and existing tests passed.

[tim-lai]

@Pompette Thanks for the PR! This proposed change also removes the auto-population of a generated sample schema even when it is "real" and intended.

I think a more accurate fix would be to check if the value of sampleSchema is "real", e.g. not string, then addItem or description as appropriate.