Fix a crash when adding a default command handler

It is allowed to call ComponentManager::AddCommandHandler with
empty component and command name to indicate that we are installing
a default command handler. The CHECK() condition should be only
verified if a specific command handler is specified.

Change-Id: Ib4c0f633e5e4ddc396a4f5030a6ebd4e411f911e
Reviewed-on: https://weave-review.googlesource.com/1820
Reviewed-by: Vitaly Buka <vitalybuka@google.com>