python/python3: use --no-run-if-empty xargs option

python/python3: use --no-run-if-empty xargs option

As suggested by Samuel Martin, this commit adds the option
--no-run-if-empty xargs option to the "find ... | xargs ..." logic used
in the python and python3 target-finalize hooks to remove py/pyc/pyo
files. This ensures that the command doesn't fail if there are no files
matching the pattern.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Reviewed-by: Samuel Martin <s.martin49@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>