fix(nimbus): Make name field in new UI required

Because

- Submitting a form with an empty name field in the new Nimbus UI causes a 500 error.

This commit

- Adds validation to ensure the name field is required and prevents submission with an empty value

Fixes #12016

experimenter/experimenter/nimbus_ui_new/tests/test_forms.py

@@ -383,6 +383,7 @@ def test_name_field_is_required(self):
         documentation_link = NimbusDocumentationLinkFactory.create()
 
         form_data = {
+            "name": "",
             "hypothesis": "new hypothesis",
             "risk_brand": True,
             "risk_message": True,
@@ -402,6 +403,7 @@ def test_name_field_is_required(self):
         form = OverviewForm(data=form_data)
 
         self.assertFalse(form.is_valid())
+        self.assertIn("name", form.errors)
 
 
 class TestDocumentationLinkCreateForm(RequestFormTestCase):