In python3.8 on macos and in all versions on windows the multiprocessing
method is `spawn` which does not preserve class state.

Now that callers are ensuring that `value` is not `None`, we can further
tighten the contract and remove the conditional to account when `None`
is passed-in for `value`.

Additionally, add a new test vector to account for when an empty string
is passed in, which would fail `if no value`.

Now that the contract has narrowed for `utils.normalize_paths()`
and `utils.parse_comma_separated_list()`, `Option.normalize()` must
handle when the option value is either a singular value or a sequence
(i.e., `list`) of values.

The paths where `Option.normalize()` is called are:

1. options/config.py: `MergedConfigParser.parse_*_config()`
    There are 3 paths wehre eventually `_normalize_value()` is called.
2. options/manager.py: `OptionManager.parse_args()`

For (1), values are coming from the `configparser` module.  For (2),
values are coming from `optparse.OptionParser`.

Rudimentary investigation seems to implicate that
`optparse.OptionParser.parse_args()` always returns values in a `list`
because it accumulates values.  However, for `configparser`, the values
are a string due to the key-value nature of the INI format.

Given that `Option.normalize()` is the convergence point where
normalization of an option occurs, it is acceptable for the method to
also handle the parsing comma-separated values and path normalization by
the option value's type.

The `normalize_paths()` utility was doing too much — parsing
unstructured configuration path data and dispatching the scrubbed paths
to be normalized.

Towards moving the parsing of unstructured configuration path data
closer towards were configuration occurs, have the utility accept only
structured input for normalizing paths.

This is a separate commit so it can be dropped during a rebase or revert
independently.

This is the initial incision point to only accept `str` (or `None`) for
parsing out comma/whitespace/regexp separated values.

This is a separate commit so it can be dropped during a rebase or
reverted independently.