Make JDA Utils only set Activity when set.

[Andre601]

Pull Request

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• [x] I have checked the pull request page for upcoming or merged features/bug fixes.
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Pull Request Information

Check and fill in the blanks for all that apply:

• [x] My PR fixes a bug, error, or other issue with the library's codebase.
• [x] My PR is for the command module of the JDA-Utilities library.
• [ ] My PR creates a new module for the JDA-Utilities library: ______.

Description

By default does JDA Utilities set the Activity no matter what. When providing null does it still set the activity which results in no activity being set at all.

This is annoying, especially considering that this is done in the ReadyEvent and could have conflicts with bots that also set the Activity on the ReadyEvent.

This PR adds a simple check, that will JDA Utils only set the Activity when the provided activity isn't null. This allows the usage of the Command package while still using their own method(s) to set a bots activity.