Standardize `cpu_freq()` null values

[giampaolo]

When min/max CPU frequency cannot be determined the value is sometimes set to None, other times to 0.0. Figure this out so that it's consistent across all platforms.

[omprakash2929]

Hi @giampaolo, I looked into this across all platforms:

• Windows (_pswindows.py): hardcodes min_ = 0.0
• Linux (_pslinux.py, fallback path when only /proc/cpuinfo is
  available): hardcodes both to 0.0 - docstring even says
  "are set to 0"
• OpenBSD (_psbsd.py): hardcodes both to 0.0
• FreeBSD (_psbsd.py): already correctly uses None when parsing
  fails - no change needed here
• NetBSD: cpu_freq() isn't implemented at all (separate issue,
  out of scope here)

The aggregation logic in __init__.py's cpu_freq() already expects
None for undetermined min/max (see the FreeBSD-related comment
there), so None looks like the intended standard.

I want to standardize Windows, Linux (fallback path), and OpenBSD to
return None instead of 0.0 for undetermined values. Should I
proceed with this approach?