Fix nullness warning in TryParseFeature by using box before downcast

Changed the null check pattern from matching on union directly to matching on box union. This satisfies the nullness analysis by ensuring the null check happens on the boxed value before downcasting, eliminating the FS3264 nullness warning.

Fixes build error reported in comment 3675368950.

Co-authored-by: T-Gro <[email protected]>

Author: Copilot

src/Compiler/Facilities/LanguageFeatures.fs

@@ -443,10 +443,9 @@ type LanguageVersion(versionText, ?disabledFeaturesArray: LanguageFeature array)
         |> Option.bind (fun case ->
             let union = Microsoft.FSharp.Reflection.FSharpValue.MakeUnion(case, [||])
 
-            if isNull union then
-                None
-            else
-                Some(union :?> LanguageFeature))
+            match box union with
+            | null -> None
+            | obj -> Some(obj :?> LanguageFeature))
 
     override x.Equals(yobj: obj) =
         match yobj with