net: clean up redundant if branch in dial

net: clean up redundant if branch in dial

Dialer.DialContext no longer performs a redundant check on the length
of the fallback slice, because dialParallel already handles the
situation where the fallback slice is empty or nil.

Change-Id: Ibb16f4813fc55dec2939c54c10e665ff35bfe163
Reviewed-on: https://go-review.googlesource.com/c/go/+/387795
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Tobias Klauser <tobias.klauser@gmail.com>