Nulls as errors (#2394)

Github will cuck me no more.

I tried the old google machine to find if there's a way to inherit these but it was failing me.

Co-authored-by: Metal Gear Sloth <metalgearsloth@gmail.com>

Content.Client/Content.Client.csproj

@@ -8,6 +8,7 @@
     <AppendTargetFrameworkToOutputPath>false</AppendTargetFrameworkToOutputPath>
     <OutputPath>..\bin\Content.Client\</OutputPath>
     <OutputType Condition="'$(FullRelease)' != 'True'">Exe</OutputType>
+    <WarningsAsErrors>CS8604;CS8765</WarningsAsErrors>
   </PropertyGroup>
   <Import Project="..\RobustToolbox\MSBuild\Robust.DefineConstants.targets" />
   <ItemGroup>