Can't run DAST scan without repository

When no default branch is provided to the on-demand scans form, we
assume that no repository has been created yet and we show an error
message to recommend creating one.

ee/app/assets/javascripts/on_demand_scans/components/on_demand_scans_form.vue

@@ -439,6 +439,13 @@ export default {
             searchPlaceholder: __('Search'),
           }"
         />
+        <div v-if="!defaultBranch" class="gl-text-red-500 gl-mt-3">
+          {{
+            s__(
+              'OnDemandScans|You must create a repository within your project to run an on-demand scan.',
+            )
+          }}
+        </div>
       </gl-form-group>
 
       <scanner-profile-selector

ee/changelogs/unreleased/324777-dast-scan-blank-project.yml

+---
+title: Call out the inability to run on-demand scans without a repository being created
+merge_request:
+author:
+type: changed

ee/spec/frontend/on_demand_scans/components/on_demand_scans_form_spec.js

@@ -630,6 +630,26 @@ describe('OnDemandScansForm', () => {
     });
   });
 
+  describe('when no repository exists', () => {
+    beforeEach(() => {
+      mountShallowSubject({
+        propsData: {
+          /**
+           * The assumption here is that, if a default branch is not defined, then the project
+           * does not have a repository.
+           */
+          defaultBranch: '',
+        },
+      });
+    });
+
+    it('shows an error message', () => {
+      expect(subject.text()).toContain(
+        'You must create a repository within your project to run an on-demand scan.',
+      );
+    });
+  });
+
   describe('dastBranchSelection feature flag disabled', () => {
     describe.each`
       action      | actionFunction | runAfter

locale/gitlab.pot

@@ -21560,6 +21560,9 @@ msgstr ""
 msgid "OnDemandScans|You cannot run an active scan against an unvalidated site."
 msgstr ""
 
+msgid "OnDemandScans|You must create a repository within your project to run an on-demand scan."
+msgstr ""
+
 msgid "Once a project is permanently deleted, it %{strongStart}cannot be recovered%{strongEnd}. Permanently deleting this project will %{strongStart}immediately delete%{strongEnd} its repositories and %{strongStart}all related resources%{strongEnd}, including issues, merge requests etc."
 msgstr ""