test(platform-server): add SSR integration test for null and normal input values

Add an Angular SSR integration test in platform-server verifying that null input values do not render string attributes like value=null during server-side rendering, while normal non-empty string values like value=hello are properly preserved.

Fixes angular#69785

Author: surajy93

packages/platform-server/test/full_app_hydration_spec.ts

@@ -211,6 +211,30 @@ describe('platform-server full application hydration integration', () => {
         expect(ssrContents).not.toContain(extraChildNodes);
         expect(ssrContents).toContain('<app ngh="0"><div>Some content</div></app>');
       });
+
+      it('should serialize input values correctly for both null and normal non-empty values during SSR', async () => {
+        @Component({
+          selector: 'app',
+          template: `
+            <input id="input-null" [value]="nullValue" />
+            <input id="input-normal" [value]="normalValue" />
+          `,
+        })
+        class AppComponent {
+          nullValue: string | null = null;
+          normalValue = 'hello';
+        }
+
+        const html = await ssr(AppComponent);
+        const ssrContents = getAppContents(html);
+
+        // A `null` value should not be reflected as a `value` attribute at all.
+        expect(ssrContents).toContain('<input id="input-null">');
+        expect(ssrContents).not.toContain('value="null"');
+
+        // A normal, non-empty value should be reflected as-is.
+        expect(ssrContents).toContain('<input id="input-normal" value="hello">');
+      });
     });
 
     describe('hydration', () => {