BUG: Fixed union_categoricals with unordered cats
Closes https://github.com/pandas-dev/pandas/issues/19096
# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/19097?src=pr&el=h1) Report
> Merging [#19097](https://codecov.io/gh/pandas-dev/pandas/pull/19097?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/bbfbe48e84f40c22187879b94a7039ff06b6d21a?src=pr&el=desc) will **increase** coverage by `0.02%`.
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/19097/graphs/tree.svg?token=eZ4WkYLtcO&src=pr&width=650&height=150)](https://codecov.io/gh/pandas-dev/pandas/pull/19097?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #19097      +/-   ##
==========================================
+ Coverage   91.51%   91.54%   +0.02%     
==========================================
  Files         148      148              
  Lines       48754    48757       +3     
==========================================
+ Hits        44619    44634      +15     
+ Misses       4135     4123      -12
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `89.91% <100%> (+0.02%)` | :arrow_up: |
| #single | `41.61% <50%> (-0.01%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/19097?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/dtypes/concat.py](https://codecov.io/gh/pandas-dev/pandas/pull/19097/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZHR5cGVzL2NvbmNhdC5weQ==) | `99.14% <100%> (+0.01%)` | :arrow_up: |
| [pandas/plotting/\_converter.py](https://codecov.io/gh/pandas-dev/pandas/pull/19097/diff?src=pr&el=tree#diff-cGFuZGFzL3Bsb3R0aW5nL19jb252ZXJ0ZXIucHk=) | `66.95% <0%> (+1.73%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/19097?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/19097?src=pr&el=footer). Last update [bbfbe48...61fad8a](https://codecov.io/gh/pandas-dev/pandas/pull/19097?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 lgtm. ping on green.