Don't wake up to check trigger file if none is configured

A streaming replica waiting on WAL from the master will wake up every 5
seconds to check for a trigger file. This is pointless if no trigger file
has been configured.

The attached patch suppresses the timeout when there is no trigger file
configured.

A minor thing to be sure, but there was a campaign a couple years ago to
remove such spurious wake-ups, so maybe this change is worthwhile.

I noticed that the existing codebase does not have a consensus on what to
pass to WaitLatch for the timeout when the timeout isn't relevant. I picked
0, but -1L also has precedent.

Cheers,

Jeff