Allow to chain Queueable Jobs without the first one.

[Mateusz7410]

Currently, there is no option to instantiate the chain without any QueueableJob, what mean we need to first add the first one, and later use the chain() method.

In some use-cases, where we are not sure if we will spawn 0/1/2/... jobs, we need workarounds to check it. But it should allow us to do something like:

QueuableBuilder builder = Async.queueable();
if ( __something__ ) {
    builder.chain(new Job1());
} 
if ( __something__ ) {
    builder.chain(new Job2());
}
if ( __something__ ) {
    builder.chain(new Job3());
}
builder.enqueue(); // should either enqueue the chain or do nothing if none added.