Closes #14088

This PR addresses #14088 by adding an additional check on the validator to ensure the input does not contain spaces.

Steps to test

1. Start JabRef
2. Open preferences
3. Open 'Entry Types'
4. Attempt to create a custom entry type with a name containing a space
5. Observe system preventing the creation of the entry type

Before:

After:

Mandatory checks

• I own the copyright of the code submitted and I license it under the MIT license
• I manually tested my changes in running JabRef (always required)
• [/] I added JUnit tests for changes (if applicable)
• I added screenshots in the PR description (if change is visible to the user)
• I described the change in CHANGELOG.md in a way that is understandable for the average user (if change is visible to the user)
• [/] I checked the user documentation: Is the information available and up to date? If not, I created an issue at https://github.com/JabRef/user-documentation/issues or, even better, I submitted a pull request updating file(s) in https://github.com/JabRef/user-documentation/tree/main/en.