Don't raise warning on merging int and float with nan
Closes https://github.com/pandas-dev/pandas/issues/20998
# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/21011?src=pr&el=h1) Report
> Merging [#21011](https://codecov.io/gh/pandas-dev/pandas/pull/21011?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/17a0ca172c5fbe3095644d6a491d3f11b142f961?src=pr&el=desc) will **not change** coverage.
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/21011/graphs/tree.svg?height=150&width=650&token=eZ4WkYLtcO&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/21011?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master   #21011   +/-   ##
=======================================
  Coverage   91.82%   91.82%           
=======================================
  Files         153      153           
  Lines       49491    49491           
=======================================
  Hits        45446    45446           
  Misses       4045     4045
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.22% <100%> (ø)` | :arrow_up: |
| #single | `41.85% <0%> (ø)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/21011?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/reshape/merge.py](https://codecov.io/gh/pandas-dev/pandas/pull/21011/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvcmVzaGFwZS9tZXJnZS5weQ==) | `94.25% <100%> (ø)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/21011?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/21011?src=pr&el=footer). Last update [17a0ca1...2c24590](https://codecov.io/gh/pandas-dev/pandas/pull/21011?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 thanks @jorisvandenbossche 