Fix: Reject first/last name that contain only spaces

What does this MR do and why?

On the registration page, frontend restricts that first name and last name cannot be empty, but this can be skipped by a space.

This MR is to make the check stricter: there must be non-whitespace characters to be considered legal.

Example of name input	Before	After
Washington	legal	legal
George Washington	legal	legal
George Washington	legal	legal
null	illegal	illegal
(only whitespace)	legal	illegal

Screenshots or screen recordings

Before	After

Questions

Do we need to update the users model, restricting first name and last name to not be empty?

This is a huge amount of work because:

1. Existing old users, many of which have empty last names
2. When registering an account using a third-party platform, the last name cannot be obtained

There are many problems with our system regarding first name and last name. The PM of the JH team is discussing how to solve them. This MR just optimizes the registration experience, and does not conflict with other problems to be resolved

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cc @prajnamas @zhzhang93 @JeremyWuuuuu