[5.6] Fix mailables always being queued for later if using Queueable trait

[jwensley2]

Currently if a Mailable uses the \Illuminate\Bus\Queueable trait, which is the default when using make:mail, it will always end up calling \Illuminate\Mail\Mailable::later because it is only checking if the delay property exists and not whether it is actually set.

This is a problem when using a 3rd part queue driver like https://github.com/shiftonelabs/laravel-sqs-fifo-queue which doesn't allow things to be queued for later since sqs fifo queues do not support it.

This fixes that problem by checking whether delay is actually set.