File Dropzone: Make message customizable

[austinoneil]

Description

Make filezone messages customizable. Also fixed a minor issue with a "null" class rendering.

References #1741

Type of change

• [x] Bug fix (non-breaking change which fixes an issue)
• [x] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to not work as expected)
• [ ] Documentation update

How Has This Been Tested?

Added unit tests.

Checklist

• [x] My code follows the style guidelines of this project
• [x] I have performed a self-review of my own code
• [x] I have commented my code, particularly in hard-to-understand areas
• [x] I have made corresponding changes to the documentation
• [x] My changes generate no new warnings
• [x] I have added tests that prove my fix is effective or that my feature works
• [x] New and existing unit tests pass locally with my changes
• [x] Any dependent changes have been merged and published in downstream modules
• [x] I have checked my code and corrected any misspellings

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[coliff]

Works good! https://deploy-preview-2388--moduswebcomponents.netlify.app/?path=/story/components-file-dropzone--default&args=instructions:Upload+Kitten+Pictures 👍