[6.x] Reconnect missing connection when beginning transaction

[cmorbitzer]

This PR adds a check that a connection is not missing before beginning a transaction, and reconnects if it is. This is the same check added in 143f7a9 that is performed before running a query in Illuminate\Database\Connection::run().

I ran into this while running a Laravel Excel queued import on Vapor. Vapor disconnects all database connections at the end of each queued job execution in QueueHandler::terminate(). When the worker executes the next job, app('db.connection')->getPdo() returns null unless the connection is re-established, which currently only happens when running a query. So when the ReadChunk queued job in Laravel Excel attempts to begin a transaction before running a query here, $this->getPdo()->beginTransaction() in ManagesTransactions results in the error Call to a member function beginTransaction() on null.