Check that the user exists before trying to fill the request

[snipe]

This better handles trying to PATCH or PUT on a user via the API where that user ID does not exist.

[what-the-diff]

PR Summary

• Improved Security Features in the User Management System
  Various rules have been implemented to ensure no unwanted changes are performed. This includes preventing users from changing administrative-level information in the demo use-case and any possible updates to user information through the API while in demo mode. Moreover, steps have been taken to prevent users from making themselves their own manager, preserving the defined system hierarchy.

• Streamlined Location and Group Updates
  The system has been programmed to automatically update the location of the resources linked to a specific user whenever any alteration to user data is made. If the user is identified as a superuser and there are specifications regarding group changes in the request, the changes will be done automatically.

• Enhanced User Customization
  Changes have been made to allow users to upload images for their avatar. This will provide users with added customization options and enhance the overall user experience.

• Improved Response Handling
  Additional responses have been added to handle various situations, such as successful updates, save errors, and instances where user data could not be found. This will improve the system's ability to alert the user about the status and outcome of their actions promptly.

• Code Refactoring and Formatting
  The code has been optimized and reformatted for better readability, easy maintenance, and improved performance.

[snipe]

Not sure how this would have broken tests exactly...