Merge pull request #30843 from cston/30840

Skip NullableWalker below #nullable disable

src/Compilers/CSharp/Portable/FlowAnalysis/DataFlowPass.cs

@@ -375,7 +375,7 @@ public static void Analyze(CSharpCompilation compilation, MethodSymbol member, B
                 walker.Free();
             }
 
-            if (member.NonNullTypes != null &&
+            if (member.NonNullTypes == true &&
                 compilation.LanguageVersion >= MessageID.IDS_FeatureNullableReferenceTypes.RequiredVersion())
             {
                 NullableWalker.Analyze(compilation, member, node, diagnostics);