Github: undefined array key warning when updating repo with empty client id/secret

Created by: dregad

When updating a Github repo with empty client id/secret (i.e. after its initial creation, when mantis_plugin_Source_repository_table.info = 'a:0:{}'), PHP issues the following warnings:

SYSTEM WARNING: 'Undefined array key "hub_app_client_id"' in '.../plugins/source-integration/SourceGithub/SourceGithub.php' line 439 SYSTEM WARNING: 'Undefined array key "hub_app_secret"' in '/.../plugins/source-integration/SourceGithub/SourceGithub.php' line 440

Then depending on MantisBT display error settings, repo_update_page.php just dies there instead of properly redirecting.