Avoid construction of unused RtcEventLogNullImpl object
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-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#19686}

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