Check if URI is valid in Import::ValidateRemoteGitEndpointService

What does this MR do and why?

Prevent Import::UrlController#validate from raising errors when an invalid URL is provided.

Related to: Fix Import::UrlController#validate errors (#388069 - closed)

Screenshots or screen recordings

Screenshots are required for UI changes, and strongly recommended for all other merge requests.

How to set up and validate locally

1. Open /projects/new#import_project
2. Select "Repo by URL"
3. Check if the Import::UrlController#validate endpoint doesn't return errors when providing a repository URL

Numbered steps to set up and validate the change are strongly suggested.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.