All function to deal with np.nan

[jbesomi]

(Edit)

As it is the case with Pandas, every texthero function should deal with not assigned (NA) values.

The rule of thumb should be the following:

If the given input Pandas Series s has some NA at indexes X, the same output Pandas Series should have the same nan at indexes X. ("NA rule")

This task consists in:

1. Add a test_na.py file that checks for every "texthero" function that the NA rule is respected
2. Change any functions that fail the NA rule (see Implement & test handling of NaNs in preprocessing with handle_nans decorator #130 ) for some input

This task has appeared to be more complex than initially thought.

Linked PR: #86, #130 and #123

[mk2510]

i will start working on this issue together with @henrifroese