Do not extract keys from empty fields.

[alexnj]

Per documentation at https://uppy.io/docs/aws-s3/, fields can be empty.
"For presigned PUT uploads, this should be left empty."

Adding a safety check before attempting to extract keys.

[arturi]

Thank you for the PR!

We are doing something different for xhr-uploads, I wonder if we need to mimic that in aws-s3 then, @goto-bus-stop:

uppy/packages/@uppy/xhr-upload/src/index.js

Lines 175 to 178 in 36604a2

	const metaFields = Array.isArray(opts.metaFields)
	? opts.metaFields
	// Send along all fields by default.
	: Object.keys(file.meta)

[goto-bus-stop]

We should not send file metadata to S3 in form fields (like XHRUpload does), so the empty array is the correct approach 👍

[arturi]

Thanks!