PERF: improve speed of nans in CategoricalIndex
This is a minor follow-up to #21369.

```python
>>> n = 100_000
>>> ci = pd.CategoricalIndex(['a']*n + ['b']*n + ['c']*n + [np.nan])
>>> np.nan in ci
19.5 us  # master
114 ns  # this PR
```

Using ``self.hasnans`` to check for nans is faster than ``self.isna().any()`` because  it's cached.

# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/21493?src=pr&el=h1) Report
> Merging [#21493](https://codecov.io/gh/pandas-dev/pandas/pull/21493?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/bf1c3dc253c08e70c543ae09c146b72544e85df5?src=pr&el=desc) will **increase** coverage by `<.01%`.
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/21493/graphs/tree.svg?height=150&width=650&src=pr&token=eZ4WkYLtcO)](https://codecov.io/gh/pandas-dev/pandas/pull/21493?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #21493      +/-   ##
==========================================
+ Coverage   91.89%   91.89%   +<.01%     
==========================================
  Files         153      153              
  Lines       49604    49604              
==========================================
+ Hits        45584    45586       +2     
+ Misses       4020     4018       -2
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.3% <100%> (ø)` | :arrow_up: |
| #single | `41.88% <0%> (ø)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/21493?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/indexes/category.py](https://codecov.io/gh/pandas-dev/pandas/pull/21493/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy9jYXRlZ29yeS5weQ==) | `97.09% <100%> (ø)` | :arrow_up: |
| [pandas/util/testing.py](https://codecov.io/gh/pandas-dev/pandas/pull/21493/diff?src=pr&el=tree#diff-cGFuZGFzL3V0aWwvdGVzdGluZy5weQ==) | `84.81% <0%> (+0.2%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/21493?src=pr&el=continue).
> **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta)
> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/21493?src=pr&el=footer). Last update [bf1c3dc...4b868bf](https://codecov.io/gh/pandas-dev/pandas/pull/21493?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 thanks @topper-123 