`MatchRules().ByBody()` NullReferenceException when the `recorded` body is not empty, and the `received` body is an empty string

[jhonny-apiiro]

Steps to reproduce:

1. Configure a cassette to match ByBody
2. Record a request (either POST or GET) on the cassette which has a non-empty Body content
3. Replay the cassette, and execute a request where the Body content is an empty string (for example, the refit library would do that when there's a request with no Body specifically defined)
   Expected: The requests do not match
   Actual: Null reference exception