Improve handling of font license errors

[mikachan]

This PR improves the handling of errors caused by the font license functionality.

• Checks if the font-credits form data is set and is an array before running any further code
• Sets an empty object {} as the default state for the font-credits data
• Checks that the font file has loaded successfully before setting the font-credits data
• Ensures all Google fonts are served over https, to prevent mixed content errors

To test:

1. Add a Google font to your theme (e.g. IBM Plex Mono)
2. Add multiple font variants (e.g. 2 different font weights)
3. The plugin should not crash or error
4. Check that the font license has been added to the theme readme.txt correctly

[madhusudhand]

This is working as expected. 🚢

Few unrelated issues found while testing.

• It leaves additional spaces for removed fonts.

• Gives this error when a font is picked from the list.

• And font-face.js (line 34) throws error because of tabindex instead of tabIndex. React context.

[mikachan]

Thanks @madhusudhand!

I've created issues for those extra tasks:

• Font removal: additional spaces are left in readme.txt from font license #361
• Google fonts: React warning when font is selected from font list #362
• Fonts: font-face.js tabindex React error #363