Changed nullable back to enable.

src/AutoRest.CSharp.V3/AutoRest.CSharp.V3.csproj

@@ -3,7 +3,7 @@
   <PropertyGroup>
     <OutputType>Exe</OutputType>
     <TargetFramework>netcoreapp3.0</TargetFramework>
-    <Nullable>annotations</Nullable>
+    <Nullable>enable</Nullable>
     <RunAnalyzersDuringBuild>true</RunAnalyzersDuringBuild>
     <RunAnalyzersDuringLiveAnalysis>false</RunAnalyzersDuringLiveAnalysis>
   </PropertyGroup>