`fetch()`'s `input` argument is not optional

[Peeja]

It's not optional in the browser DOM signature which this is emulating, and it's not clear what the behavior of a fetch() should be if it were given undefined. Type-wise, making this optional requires any client code providing a fetch() function to handle the undefined value, even though it's nonsensical.

See also https://github.com/comunica/comunica/pull/1233: Comunica's fetch() implementations don't accept undefined, but it wasn't breaking because "strictFunctionTypes": false was ignoring the errors.

[coveralls]

Pull Request Test Coverage Report for Build 5601690892

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 99.187%

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Totals
Change from base Build 5387797736:	0.0%
Covered Lines:	87
Relevant Lines:	88

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💛 - Coveralls

[rubensworks]

Thanks! Released as 4.0.0.