Return null instead of NaN for unit config key names

[spokodev]

parse.unit keeps the group, decimal and placeholder config values on the same object as the unit multipliers. When an input token matches one of those names, parse.unit[units] resolves to a string (e.g. ',') instead of undefined, so n * units evaluates to NaN and the function returns NaN rather than null:

parse('1group')       // NaN  (expected null)
parse('1decimal')     // NaN  (expected null)
parse('1placeholder') // NaN  (expected null)

Only accumulate a component when the resolved unit is a number, so config-key names are treated as unknown units.