Avoid construction of unused RtcEventLogNullImpl object

This CL addresses to nits:
1. When event_log_factory_ is not null, there's no need to create a RtcEventLogNullImpl object.
2. Use MakeUnique.

BUG=None

Review-Url: https://codereview.webrtc.org/3012763002
Cr-Original-Commit-Position: refs/heads/master@{#19686}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: 393a9f6926636b94ec8accd5dfe4cd6e5c8ebafe