Telemetry Frequency incorrectly affects before and after fuzzing telemetry collection

Per [ centipede_flags.inc](https://github.com/google/fuzztest/blob/main/centipede/centipede_flags.inc#L251), before-fuzzing and after-fuzzing telemetry data should always be dumped, regardless of the value of the `telemetry_frequency` flag. However, right now, this is only partially true. Corpus telemetry is not dumped if `telemetry_frequency` is zero, while RUsage telemetry is.

This occurs because [`DumpCorpusTelemetryInThisShard`](https://github.com/google/fuzztest/blob/main/centipede/environment.cc#L74) checks `telemetry_frequency` and requires it to be non-zero, while [`DumpRUsageTelemetryInThisShard`](https://github.com/google/fuzztest/blob/main/centipede/environment.cc#L79) does not make this check.

`DumpCorpusTelemetryInThisShard` should be updated to remove the requirement that `telemetry_frequency` is non-zero.