fix: requestBodyFunc handles functions returning nil readers; add RetrieveReaderFromRequestBody

[bmoylan]

Fixes failures like this conjure-go test failure: https://app.circleci.com/pipelines/github/palantir/conjure-go/1963/workflows/9d9e8d4a-91e2-4ced-8e6d-dcbef1aca645/jobs/7973/steps

=== RUN   TestBytes/BinaryOptionalAlias_empty
    binary_test.go:112: 
                Error Trace:    /home/circleci/go/src/github.com/palantir/conjure-go/integration_test/testgenerated/binary/binary_test.go:112
                Error:          Received unexpected error:
                                http: Request.ContentLength=-1 with nil Body
                                httpclient request failed map[requestHost:127.0.0.1:33297 requestMethod:Post]
...

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This change is 

[autorelease3]

Released v2.85.0