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.Shared/Content.Shared.csproj

@@ -9,6 +9,7 @@
     <OutputPath>../bin/Content.Shared</OutputPath>
     <Configurations>Release;Debug</Configurations>
     <Platforms>AnyCPU</Platforms>
+    <WarningsAsErrors>CS8604;CS8765</WarningsAsErrors>
   </PropertyGroup>
   <Import Project="..\RobustToolbox\MSBuild\Robust.DefineConstants.targets" />
   <ItemGroup>