oeqa/utils/commands.py: Command class improve validations/decoding in output

oeqa/utils/commands.py: Command class improve validations/decoding in output

When run a command sometimes the output isn't provided so validate
before trying to encode to utf-8, also some output like BIOS/EFI
contains characters that can't be codified into utf-8 for this reason
set errors='replace'.

[YOCTO #10019]

Signed-off-by: Aníbal Limón <anibal.limon@linux.intel.com>
Signed-off-by: Ross Burton <ross.burton@intel.com>