trace: Don't report cpu_frequency if no events from kernel

Created by: bjackman

devlib always introduces fake cpu_frequency events (cpu_frequency_devlib). If the kernel doesn't produce any 'real' cpu_frequency events the current behaviour is to use only the devlib ones.

This is misleading as it gives the impression that we have frequency data where we do not. Instead, if there are no events from the kernel, don't report any frequency data.