Special use-case:
PasswordFormMetricsRecorder owns a UkmEntryBuilder, and ends up passing an entry to a UkmRecorder. In some cases, the builder needs to be discarded and replaced by another one. The discarded builder has collected no metrics, so it would be helpful if it did not leave any effect on what is uploaded.

While we can work around that in PasswordFormMetricsRecorder, in general, not adding an empty entry to the upload looks like it does not make the metrics less useful.

Metrics team, what do you think? Please assign to vabr@ once you decide if this looks good to you or not.