Fix WithBody when using Pact and added more nullable annotations

[StefH]

No description provided.

[codecov]

Codecov Report

Merging #783 (c31e5fb) into master (b1af37f) will increase coverage by 0.08%.
The diff coverage is 73.87%.

@@            Coverage Diff             @@
##           master     #783      +/-   ##
==========================================
+ Coverage   71.97%   72.06%   +0.08%     
==========================================
  Files         209      210       +1     
  Lines        7280     7321      +41     
  Branches      753      762       +9     
==========================================
+ Hits         5240     5276      +36     
- Misses       1810     1815       +5     
  Partials      230      230              

Impacted Files	Coverage Δ
...k.Net.Abstractions/Admin/Mappings/WebProxyModel.cs	0.00% <0.00%> (ø)
....Matchers.CSharpCode/Matchers/CSharpCodeMatcher.cs	83.33% <ø> (ø)
src/WireMock.Net/Compatibility/EmptyArray.cs	0.00% <0.00%> (ø)
...WireMock.Net/HttpsCertificate/CertificateLoader.cs	0.00% <0.00%> (ø)
...ck.Net/HttpsCertificate/PublicCertificateHelper.cs	0.00% <0.00%> (ø)
...ireMock.Net/ResponseBuilders/Response.WithProxy.cs	100.00% <ø> (ø)
.../ResponseProviders/DynamicAsyncResponseProvider.cs	0.00% <0.00%> (ø)
...reMock.Net/Server/WireMockServer.ConvertMapping.cs	43.75% <0.00%> (ø)
...rc/WireMock.Net/Settings/WireMockServerSettings.cs	94.59% <0.00%> (-2.63%)	⬇️
...eMock.Net/Settings/WireMockServerSettingsParser.cs	0.00% <0.00%> (ø)
... and 74 more

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