Scheduler settings cannot be migrated due to RequirementsException: No database connection available for source plugin variable

When running migrations, the Scheduler module tries to use the 'variable' source plugin, which requires a legacy database connection.
If this connection is not available, a RequirementsException is thrown, causing a notice during cache rebuild or other operations.

This solution adds a minimal check for the legacy database connection before attempting to create the stub migration.
If the connection is missing, the migration alteration early-exits gracefully, preventing the exception and related log notices.

This avoids cluttering logs and errors in environments where the legacy database is not present or migrations are not needed,
without changing the Scheduler migration functionality when the connection is available.

Hi antonio nunez,
Thanks for raising this, and for providing the MR. I have to admit that someone else wrote the Scheduler migration process, and so I do not have an in-depth knowledge of all the details, but if you have found the situation where there is no database connection then yes I agree that simple check and early exit is the correct thing to do. I have not been able to replicate this error. It is clearly not as simple as the two steps you list in the issue summary. How exactly did you get the scenario of not having the database connection?