Add test suite forNumCharactersand modify docstring

[sbadithe]

• Fixes NumCharacters does not have test suite  #2462
• Refactors NumCharacters to return null given null input

[sbadithe]

Changing NumCharacters to return IntegerNullable triggered an error here. Int64 doesn't seem to support the corr operation. There is a _is_integer_dtype function, but that would cast a wider net and grab series with the int64 dtype. This way is a little more selective and does the conversion less often.

[codecov]

Codecov Report

Merging #2463 (3f082bf) into main (1f04ed3) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main    #2463   +/-   ##
=======================================
  Coverage   99.42%   99.42%           
=======================================
  Files         354      355    +1     
  Lines       21385    21421   +36     
=======================================
+ Hits        21261    21297   +36     
  Misses        124      124           

Impacted Files	Coverage Δ
...ools/tests/primitive_tests/test_primitive_utils.py	100.00% <ø> (ø)
...ndard/transform/natural_language/num_characters.py	100.00% <100.00%> (ø)
featuretools/selection/selection.py	100.00% <100.00%> (ø)
...l_language_primitives_tests/test_num_characters.py	100.00% <100.00%> (ø)

Help us with your feedback. Take ten seconds to tell us how you rate us. Have a feature suggestion? Share it here.

[thehomebrewnerd]

This looks good to me. My only minor nitpick is that is unrelated to this change, is that perhaps we should update the primitive docstring to make it more clear that this primitive counts all characters, including whitespace and punctuation, but that's not a big deal.