Fix StringChecker Softlock bug

[RiversJohn]

• Fix StringChecker bugs

• - I confirm this PR is at its final form and will not receive more pushes to it unless modifications are required.

• - I confirm this PR has been previously tested by me and has no apparent issues.

• - I confirm this PR is complying with this project's Contribution Guidelines.

• - I confirm this PR is complying with this project's Syntax Ruleset.

• This PR fixes a possible softlock state that could be caused by the user submitting IllegalChar's, the Checker function can slip whitespaces due to using string.Contains improperly

  • If the user did submit an illegal username, the server then executes the same function for the username and finds it invalid due to comparing with a different CultureInfo where said whitespace will in fact match
  • This results in the user being unable to login anywhere without first using the mod options menu delete account option

• PR also removes a superfluous call to string.IsNullOrEmpty, string.IsNullOrWhitespace already checks against string.Empty as can be seen in it's XML summary explicitly stated.