fix: set parameter to string in explode function

[pplulee]

Fixes FriendsOfFlarum/user-directory#109

The issue can be reproduced in PHP8.x environment when using the extension FriendsOfFlarum/user-directory. Issue is caused by passing null parameter into function str_getcsv() in Flarum Core, where PHP8.x doesn't allow that.

Issue is solved by set the parameter to string.

Changes proposed in this pull request: GambitManager.php

Necessity

• [ ] Has the problem that is being solved here been clearly explained?
• [ ] If applicable, have various options for solving this problem been considered?
• [ ] For core PRs, does this need to be in core, or could it be in an extension?
• [ ] Are we willing to maintain this for years / potentially forever?

Confirmed

• [ ] Frontend changes: tested on a local Flarum installation.
• [ ] Backend changes: tests are green (run composer test).
• [ ] Core developer confirmed locally this works as intended.
• [ ] Tests have been added, or are not appropriate here.

Required changes:

• [ ] Related documentation PR: (Remove if irrelevant)
• [ ] Related core extension PRs: (Remove if irrelevant)

[SychO9]

Hello @pplulee thanks for the PR!

The 1.x branch is only currently receiving critical patches. This is a very minor issue and involves improving your web server configuration by disabling deprecation warnings in production to avoid the likely numerous similar warnings.

I would redirect your PR to 2.x, but the 2.x code has changed a lot and likely this is solved there.

Thanks again! Feel free to send more PRs in the future.

[luceos]

I've confirmed this is not currently an issue in 2.x.

Thank you for your contribution, please suppress deprecations and warnings through your php.ini or using ini_set inside the config.php to go around this issue.