issue-177204: show username when fullName is empty string (#186298)

## Summary

We could find and edge case on ESS Custom User doing QA on the issue:
#177204.
When user's full name is an empty string (not undefined or null) the
greeting message was not displaying.

In this PR, we change the logic and display user's username whenever the
user's fullName value is an empty string.

Before:

![image](https://github.com/elastic/kibana/assets/61565784/9138a923-647c-4c6d-9300-340040545e55)


How it looks now:

![image](https://github.com/elastic/kibana/assets/61565784/d451ed60-cdcf-41f1-a01a-12f5d0c4202a)


### **Checklist**


- [ ] [Unit or functional
tests](https://www.elastic.co/guide/en/kibana/master/development-tests.html)
were updated or added to match the most common scenarios.

x-pack/plugins/security_solution/public/common/components/landing_page/onboarding/welcome_header/index.test.tsx

@@ -42,6 +42,16 @@ describe('WelcomeHeaderComponent', () => {
     expect(titleElement).toBeInTheDocument();
   });
 
+  it('should render username when fullName is an empty string', () => {
+    const fullName = '';
+    const username = 'jd';
+    mockUseCurrentUser.mockReturnValue({ fullName, username });
+
+    const { getByText } = render(<WelcomeHeader />);
+    const titleElement = getByText(`Hi ${username}!`);
+    expect(titleElement).toBeInTheDocument();
+  });
+
   it('should render username when fullName is not provided', () => {
     const username = 'jd';
     mockUseCurrentUser.mockReturnValue({ username });

x-pack/plugins/security_solution/public/common/components/landing_page/onboarding/welcome_header/index.tsx

@@ -24,7 +24,7 @@ const WelcomeHeaderComponent: React.FC<{ productTier?: ProductTier }> = ({ produ
   const userName = useCurrentUser();
 
   // Full name could be null, user name should always exist
-  const name = userName?.fullName ?? userName?.username;
+  const name = userName?.fullName || userName?.username;
 
   const projectFeaturesUrl = useProjectFeaturesUrl();