This prepares for simplifying the behavior of optionals so that
an empty parameter value resets the optional.
Bug: webrtc:9883
Change-Id: I8ef8fe9698235044cac66bc4a587abe874c8f854
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/150883
Commit-Queue: Sebastian Jansson <srte@webrtc.org>
Reviewed-by: Björn Terelius <terelius@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#29061}