fix(migrations): avoid modifying testing modules without declarations (#48921)

Fixes that we were changing the testing modules that have no `declarations` unnecessarily, resulting in more formatting changes that users would have to clean up.

PR Close #48921

Author: crisbeto

Diff for: packages/core/schematics/ng-generate/standalone-migration/to-standalone.ts

@@ -437,6 +437,11 @@ function analyzeTestingModules(
 
   for (const obj of testObjects) {
     const declarations = extractDeclarationsFromTestObject(obj, typeChecker);
+
+    if (declarations.length === 0) {
+      continue;
+    }
+
     const importsProp = findLiteralProperty(obj, 'imports');
     const importElements = importsProp && hasNgModuleMetadataElements(importsProp) ?
         importsProp.initializer.elements :

Diff for: packages/core/schematics/test/standalone_migration_spec.ts

@@ -1043,6 +1043,34 @@ describe('standalone migration', () => {
     `));
   });
 
+  it('should not change testing objects with no declarations', async () => {
+    const initialContent = `
+      import {NgModule, Component} from '@angular/core';
+      import {TestBed} from '@angular/core/testing';
+      import {ButtonModule} from './button.module';
+      import {MatCardModule} from '@angular/material/card';
+
+      describe('bootrstrapping an app', () => {
+        it('should work', () => {
+          TestBed.configureTestingModule({
+            imports: [ButtonModule, MatCardModule]
+          });
+          const fixture = TestBed.createComponent(App);
+          expect(fixture.nativeElement.innerHTML).toBe('<hello>Hello</hello>');
+        });
+      });
+
+      @Component({template: 'hello'})
+      class App {}
+    `;
+
+    writeFile('app.spec.ts', initialContent);
+
+    await runMigration('convert-to-standalone');
+
+    expect(tree.readContent('app.spec.ts')).toBe(initialContent);
+  });
+
   it('should migrate tests with a component declared through Catalyst', async () => {
     writeFile('app.spec.ts', `
       import {NgModule, Component} from '@angular/core';