Fix empty flushed=True request for forced flushes

[basepi]

What does this pull request do?

It turns out I never actually tested the functionality for sending an empty request with flushed=True when an explicit flush is called with no data on the buffer. The function signature was wrong (😅), and we also can't POST with a None body, we need an empty dict for the body.

Luckily, it should be rare to hit this issue because we usually flush before we've had a chance to send the contents of the buffer, and that flushed=True worked fine.

This fixes those issues and adds a test for this explicit case.

Thanks to @josef-cech-acc for catching this.

Related issues

Closes #1627

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• Start Time: 2022-09-07T18:40:18.845+0000

• Duration: 31 min 57 sec

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Name	Metrics % (covered/total)	Diff
Packages	100.0% (66/66)	💚
Files	100.0% (225/225)	💚
Classes	100.0% (225/225)	💚
Lines	89.815% (17461/19441)	👍 0.004
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