fix: avoid panics on typed nil request bodies

[happysnaker]

Summary

• treat typed nil pointer bodies as no body instead of dereferencing them
• fix the panic reported for *bytes.Buffer(nil)
• add focused regression coverage for typed nil *bytes.Buffer and *bytes.Reader request bodies

Why

The current getBodyReaderAndContentLength type-switch handles concrete pointer body types like *bytes.Buffer and *bytes.Reader, but it assumes the pointer value is non-nil once the dynamic type matches. That means a typed nil body can panic during request construction instead of behaving like an absent body.

This patch treats typed nil pointer/function bodies the same way as a nil body and keeps the resulting request behavior aligned with the existing no-body path.

Closes #188.

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[happysnaker]

This should be ready for review on my side.

I have already signed the HashiCorp CLA previously, but the license/cla check is still showing as pending right after PR creation, so I'm waiting for that status to refresh.

Change summary:

• treat typed nil pointer bodies like *bytes.Buffer(nil) as no body instead of dereferencing them
• keep request construction aligned with the existing nil-body path
• add focused regression coverage for typed nil *bytes.Buffer and *bytes.Reader bodies