fix: avoid null exception when received body is empty but recorded body isn't

[jhonny-apiiro]

Firstly, thank you so much for creating and maintaining this library 🙏
I've been a VCR user in ruby, and have been looking for a comparable library for c# for a while.
Thank you for your hard work!

I encountered the issue below when using VCR in conjunction with the refit library (https://github.com/reactiveui/refit).
Refit would automatically assign an empty body to requests, unless defined otherwise.
I would then get null ref exception when trying to record a new request onto an existing cassette.

Description

Closes #58

Testing

Unit test added

Pull Request Type

Please select the option(s) that are relevant to this PR.

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Improvement (fixing a typo, updating readme, renaming a variable name, etc)

[nwithan8]

Hello, thanks for reaching out and for the kind words!

Good catch on this, we'll get it patched ASAP.