Disable download buttons

[yoonieaj]

Proposed Changes

Disable Download JSON and Download SVG buttons when there is no input or output, respectively.

This PR also fixes a bug where pressing the Download JSON button would not download the JSON input currently inside the input box.

...

before these changes, it was possible to have an empty output box and an enabled `Download SVG` button:  after these changes, this should no longer be possible:  (similarly for the `Download JSON` button as well.)

Screenshots of your changes (if applicable)

Type of Change

(Write an X or a brief description next to the type or types that best describe your changes.)

Type	Applies?
🚨 Breaking change (fix or feature that would cause existing functionality to change)
✨ New feature (non-breaking change that adds functionality)
🐛 Bug fix (non-breaking change that fixes an issue)	X
🎨 User interface change (change to user interface; provide screenshots)	X
♻️ Refactoring (internal change to codebase, without changing functionality)
🚦 Test update (change that only adds or modifies tests)
📚 Documentation update (change that only updates documentation)
📦 Dependency update (change that updates a dependency)
🔧 Internal (change that only affects developers or continuous integration)

Checklist

(Complete each of the following items for your pull request. Indicate that you have completed an item by changing the [ ] into a [x] in the raw text, or by clicking on the checkbox in the rendered description on GitHub.)

Before opening your pull request:

• [x] I have performed a self-review of my changes.
  • Check that all changed files included in this pull request are intentional changes.
  • Check that all changes are relevant to the purpose of this pull request, as described above.
• [ ] I have added tests for my changes, if applicable.
  • This is required for all bug fixes and new features.
• [ ] I have updated the project documentation, if applicable.
  • This is required for new features.
• [ ] If this is my first contribution, I have added myself to the list of contributors.
• [x] I have updated the project Changelog (this is required for all changes).

After opening your pull request:

• [x] I have verified that the CI tests have passed.
• [x] I have reviewed the test coverage changes reported by Coveralls.
• [x] I have requested a review from a project maintainer.

[coveralls]

Pull Request Test Coverage Report for Build 11026316164

Details

• 2 of 2 (100.0%) changed or added relevant lines in 2 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.02%) to 89.815%

---

Totals
Change from base Build 10973102526:	0.02%
Covered Lines:	406
Relevant Lines:	436

---

💛 - Coveralls