Treat nullable warnings as errors everywhere (#7613)

Author: glen-84

src/Directory.Build.props

@@ -4,6 +4,7 @@
     <LangVersion>preview</LangVersion>
     <Version Condition="$(Version) == ''">0.0.0</Version>
     <NoWarn>$(NoWarn);CS0436;RS0026;RS0027;RS0041;AD0001;CS1591;NU5104;NU5128;NU5501</NoWarn>
+    <WarningsAsErrors>$(WarningsAsErrors);nullable</WarningsAsErrors>
     <GenerateDocumentationFile>true</GenerateDocumentationFile>
     <ImplicitUsings>enable</ImplicitUsings>
     <Nullable>enable</Nullable>
@@ -12,7 +13,6 @@
 
   <PropertyGroup Condition="'$(CI)' == 'true'">
     <TreatWarningsAsErrors>true</TreatWarningsAsErrors>
-    <WarningsAsErrors>$(WarningsAsErrors);nullable</WarningsAsErrors>
     <WarningsNotAsErrors>NU1901;NU1902;NU1903;NU1904</WarningsNotAsErrors>
   </PropertyGroup>