ui: Avoid '[NULL]' for unnamed global tracks
ui: Avoid '[NULL]' for unnamed global tracks

Screenshot: https://screenshot.googleplex.com/5jLFDrjw3RCanKv
Bug: 179505248
Change-Id: I0afb20663d6cf33c577d846375be9c4e62b00502
google/perfetto,google/perfetto,google/perfetto,google/perfetto,google/perfetto,google/perfetto,google/perfetto,google/perfetto