Type FirstOrNull/LastOrNull as nullable instead of suppressing

These helpers are documented to return null when nothing matches, but
returned `null!`, hiding the very nullable warnings #nullable enable is
meant to surface: a miss handed back null typed as non-null and would
NRE downstream with no compile-time signal. Returning T? lets the
compiler enforce the guard at each call site (both existing callers
already null-check). GetVariable forwards the result, so it becomes
VariableInitializer? to match.

Assisted-by: Claude:claude-opus-4-8:Claude Code

ICSharpCode.Decompiler/CSharp/Syntax/AstNodeCollection.cs

@@ -294,22 +294,22 @@ namespace ICSharpCode.Decompiler.CSharp.Syntax
 		/// Returns the first element for which the predicate returns true,
 		/// or null if no such object is found.
 		/// </summary>
-		public T FirstOrNull(Func<T, bool>? predicate = null)
+		public T? FirstOrNull(Func<T, bool>? predicate = null)
 		{
 			if (list != null)
 				foreach (T item in list)
 					if (predicate == null || predicate(item))
 						return item;
-			return null!;
+			return null;
 		}
 
 		/// <summary>
 		/// Returns the last element for which the predicate returns true,
 		/// or null if no such object is found.
 		/// </summary>
-		public T LastOrNull(Func<T, bool>? predicate = null)
+		public T? LastOrNull(Func<T, bool>? predicate = null)
 		{
-			T result = null!;
+			T? result = null;
 			if (list != null)
 				foreach (T item in list)
 					if (predicate == null || predicate(item))

ICSharpCode.Decompiler/CSharp/Syntax/Statements/VariableDeclarationStatement.cs

@@ -48,7 +48,7 @@ namespace ICSharpCode.Decompiler.CSharp.Syntax
 		[Slot("Variable")]
 		public partial AstNodeCollection<VariableInitializer> Variables { get; }
 
-		public VariableInitializer GetVariable(string name)
+		public VariableInitializer? GetVariable(string name)
 		{
 			return Variables.FirstOrNull(vi => vi.Name == name);
 		}