[serial] SerialPort.setSignals() fails if empty signals dictionary

This CL makes sure SerialPort.setSignals() returns a promise that
rejects with TypeError if the signals dictionary doesn't contain at
least one signal.

Spec change: https://github.com/reillyeon/serial/pull/6

Bug: 884928
Change-Id: Ie3d17bc2fb7ea2a7a73c0d0984d57b79f60ff502
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2241540
Reviewed-by: Reilly Grant <reillyg@chromium.org>
Commit-Queue: François Beaufort <beaufort.francois@gmail.com>
Cr-Commit-Position: refs/heads/master@{#778671}