add @namespace edge case to tests

Co-authored-by: Jordan Pittman <[email protected]>

Author: RobinMalfait

packages/tailwindcss/src/ast.test.ts

@@ -141,6 +141,7 @@ it('should not emit empty rules once optimized', () => {
     @charset 'UTF-8';
     @layer foo, bar, baz;
     @custom-media --modern (color), (hover);
+    @namespace 'http://www.w3.org/1999/xhtml';
   `)
 
   expect(toCss(ast)).toMatchInlineSnapshot(`
@@ -172,13 +173,15 @@ it('should not emit empty rules once optimized', () => {
     @charset 'UTF-8';
     @layer foo, bar, baz;
     @custom-media --modern (color), (hover);
+    @namespace 'http://www.w3.org/1999/xhtml';
     "
   `)
 
   expect(toCss(optimizeAst(ast))).toMatchInlineSnapshot(`
     "@charset 'UTF-8';
     @layer foo, bar, baz;
     @custom-media --modern (color), (hover);
+    @namespace 'http://www.w3.org/1999/xhtml';
     "
   `)
 })