Use null-conditional check instead of ?? new DiagnosticBag()

Co-authored-by: CyrusNajmabadi <4564579+CyrusNajmabadi@users.noreply.github.com>

Author: Copilot

src/Compilers/CSharp/Portable/Binder/Binder_Statements.cs

@@ -709,7 +709,10 @@ private BoundStatement BindDeclarationStatementParts(LocalDeclarationStatementSy
             // Check for duplicate modifiers in local declarations.
             // We call ToDeclarationModifiers here only to report duplicate modifier errors.
             // The actual modifier (const) is determined by node.IsConst below.
-            _ = node.Modifiers.ToDeclarationModifiers(isForTypeDeclaration: false, diagnostics.DiagnosticBag ?? new DiagnosticBag());
+            if (diagnostics.DiagnosticBag is { } bag)
+            {
+                _ = node.Modifiers.ToDeclarationModifiers(isForTypeDeclaration: false, bag);
+            }
 
             var typeSyntax = node.Declaration.Type;
             bool isConst = node.IsConst;