feat: add null handling for round trip parameter and add verifying test

[TimothyMakkison]

Passing a null value into as a round tripping parameter would previously result in:

[Null Reference] Object reference not set to an instance of an object.
   at Refit.RequestBuilderImplementation.AppendDynamicRouteFragment(ValueStringBuilder& vsb, RestMethodInfoInternal restMethod, Object[] paramList, ParameterFragment fragment, RestMethodParameterInfo parameterMapValue)

• I've added a null check, when the parameter is null it will instead call StringHelpers.EscapeDataString(_settings.UrlParameterFormatter.Format()), similar to how AppendObjectPropertyFragment and AppendDynamicRouteFragment handle it. This way the user can choose how this is handled.
  • Not sure if this would be considered to be a breaking change
• Added a test verifying the behaviour