[BUG] don't mangle null-objects in value_counts
- [x] closes #42688
- [x] tests added / passed
- [x] Ensure all linting tests pass, see [here](https://pandas.pydata.org/pandas-docs/dev/development/contributing.html#code-standards) for how to run them
- [x] whatsnew entry

Null-like values are no longer mangled in `value_counts`.

This was overlooked in #22296, as decision was made not to mangle null-like values (np.nan, None, pd.NaT and so on).

It also has impact on `mode`, as it uses `value_counts` under the hood.
asv shows, what cost mangling of null-like values would have:

```
       before           after         ratio
     [9731fd07]       [d5454938]
-         174±2ms          138±2ms     0.79  series_methods.ValueCountsObjectDropNAFalse.time_value_counts(100000)
-     1.16±0.01ms         916±20μs     0.79  series_methods.ValueCountsObjectDropNAFalse.time_value_counts(1000)
-         144±2ms        111±0.6ms     0.77  series_methods.ModeObjectDropNAFalse.time_mode(100000)
-      7.03±0.1ms      4.40±0.05ms     0.63  series_methods.ModeObjectDropNAFalse.time_mode(10000)

SOME BENCHMARKS HAVE CHANGED SIGNIFICANTLY.
PERFORMANCE INCREASED.
``` This pull request is stale because it has been open for thirty days with no activity. Please update or respond to this comment if you're still interested in working on this. @mroeschke some tests are failing, but it looks as if it is not due to this PR. thanks @realead very nice!