Minor: Fix user details page bugs

[aniketkatkar97]

I worked on the following:

• [x] Fix the user avatar not showing any letter when the displayName is an empty string
• [x] Changed the patch request payload for displayName change to remove the displayName field if case of empty input instead of sending an empty string.

Before:

https://github.com/open-metadata/OpenMetadata/assets/51777795/09b96838-30ec-4d97-bc3e-272587f1f411

After:

https://github.com/open-metadata/OpenMetadata/assets/51777795/3fe8a0e2-826c-4ff1-9df1-045b74629dd4

Type of change:

• [x] Bug fix
• [x] Improvement

Checklist:

• [x] I have read the CONTRIBUTING document.
• [ ] My PR title is Fixes <issue-number>: <short explanation>
• [ ] I have commented on my code, particularly in hard-to-understand areas.
• [ ] For JSON Schema changes: I updated the migration scripts or explained why it is not needed.