samples: intel_s1000: Simplify code and fix pylint warning

Empty sequences in Python are falsy, so

    if len(config_file) != 0:

can be simplified to

    if config_file:

pylint warning:

    C1801: Do not use `len(SEQUENCE)` to determine if a sequence is
    empty (len-as-condition)

Simplify the code a bit with os.path.join(), which indirectly gets rid
of the warning. os.path.join('', 'foo') returns 'foo', so things work
out when os.path.basename() returns '' (no directory) as well.

I'm getting rid of pylint warnings for a CI check.

Also replace a '== None' with 'is None', which is more common.

Signed-off-by: Ulf Magnusson <Ulf.Magnusson@nordicsemi.no>