Checking message parts throws `StringIndexOutOfBoundsException`

[Matyrobbrt]

Issue Checklist

• I have checked for similar issues on the Issue tracker
• I have updated to the latest version of JDA-Chewtils.
• I have checked the branches or the maintainer's PRs for upcoming bug fixes.

Affected Modules

Command

Description

Currently, CommandClientImpl#getParts always expects a space after the mention:

On top of that we need to do another +1 because the default @mention prefix will always be followed by a space

However, this causes a StringIndexOutOfBoundsException when a message is just a ping without a space (for example, if the bot is pinged to make it join a thread).