When parseCpuFrequencies returns an empty vector, getCpuFrequency
would attempt to dereference an invalid iterator.
Return early from getCpuFrequency when parseCpuFrequencies returns an
empty vector.
Resolves a segfault when waybar is run within a VM on apple silicon.
On some systems (eg: ARM) the supported frequencies of the CPU are not
properly reported by /proc/cpuinfo so if that fails try to retrieve them
from /sys/devices/system/cpu/cpufreq/policy[0-9]/cpuinfo_[max|min]_freq.
