[dart2] [client] Check request .bodyBytes, not .body

[0xNF]

fix #17546

This PR is part of the effort to split PR #17548

Changes request parsing from checking .body.isNotEmpty to .bodyBytes.isNotEmpty, because .body implicitly converts to a utf-8 string, and will crash when trying to deserde data that isn't valid utf-8.

@jaumard (2018/09) @josh-burton (2019/12) @amondnet (2019/12) @sbu-WBT (2020/12) @kuhnroyal (2020/12) @agilob (2020/12) @ahmednfwela (2021/08)

[agilob]

How a byte array isn't a valid utf8 string(?)?

[kuhnroyal]

No matching octet sequence or just unknown/invalid unicode.
Makes sense to me, but can anyone test this? :)

[0xNF]

How a byte array isn't a valid utf8 string(?)?

For example, an endpoint that returns the bytes of a jpeg.
Or an endpoint that serves utf16 strings.

In my case it was image data that caused exceptions, but endpoints can serve any encoding of byte, and every encoding is not valid utf8.

[wing328]

for testing, we can consider adding echo api client for dart. We already have an endpoint to return a gif, we can create another endpoint to return a utf6 string.

@0xNF ping me if you need help adding the Dart echo api client to perform integration tests.