Return default value T() if no values have been added to the filter. Together with https://webrtc-review.googlesource.com/c/src/+/369440, DataRate etc can be used by the filter. Bug: None Change-Id: I3d0e1a3e698a91a6197bf434ace2ff8246dc393e Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/369420 Reviewed-by: Mirko Bonadei <mbonadei@webrtc.org> Reviewed-by: Danil Chapovalov <danilchap@webrtc.org> Commit-Queue: Per Kjellander <perkj@webrtc.org> Cr-Commit-Position: refs/heads/main@{#43461}