BUG, TST: fix-_check_ticks_props
Here's something I noticed while working on #34334 : `self._check_ticks_props(ax, ylabelsize=0)` always passes!

This is because of
```python
if ylabelsize
```
instead of
```python
if ylabelsize is not None
```
being used.

The test I've added fails on master:
```python-traceback
(pandas-dev) marco@marco-Predator-PH315-52:~/pandas-dev$ pytest pandas/tests/plotting/test_frame.py::TestDataFramePlots::test_plot_with_rot
============================================================== test session starts ==============================================================
platform linux -- Python 3.8.2, pytest-5.4.1, py-1.8.1, pluggy-0.13.1
rootdir: /home/marco/pandas-dev, inifile: setup.cfg
plugins: cov-2.8.1, xdist-1.31.0, asyncio-0.10.0, hypothesis-5.8.0, forked-1.1.2
collected 1 item                                                                                                                                

pandas/tests/plotting/test_frame.py .                                                                                                     [100%]

=============================================================== 1 passed in 0.33s ===============================================================  
(pandas-dev) marco@marco-Predator-PH315-52:~/pandas-dev$ git checkout upstream/master -- pandas/tests/plotting/common.py
(pandas-dev) marco@marco-Predator-PH315-52:~/pandas-dev$ pytest pandas/tests/plotting/test_frame.py::TestDataFramePlots::test_plot_with_rot
============================================================== test session starts ==============================================================
platform linux -- Python 3.8.2, pytest-5.4.1, py-1.8.1, pluggy-0.13.1
rootdir: /home/marco/pandas-dev, inifile: setup.cfg
plugins: cov-2.8.1, xdist-1.31.0, asyncio-0.10.0, hypothesis-5.8.0, forked-1.1.2
collected 1 item                                                                                                                                

pandas/tests/plotting/test_frame.py F                                                                                                     [100%]

=================================================================== FAILURES ====================================================================
_____________________________________________________ TestDataFramePlots.test_plot_with_rot _____________________________________________________

self = <pandas.tests.plotting.test_frame.TestDataFramePlots object at 0x7f8c7578f3a0>

    def test_plot_with_rot(self):
        # GH 34768
        df = pd.DataFrame({"b": [0, 1, 0], "a": [1, 2, 3]})
        ax = _check_plot_works(df.plot, rot=30)
        ax.yaxis.set_tick_params(rotation=30)
        msg = "expected 0.00000 but got "
        with pytest.raises(AssertionError, match=msg):
>           self._check_ticks_props(ax, xrot=0)
E           Failed: DID NOT RAISE <class 'AssertionError'>

pandas/tests/plotting/test_frame.py:3360: Failed
============================================================ short test summary info ============================================================
FAILED pandas/tests/plotting/test_frame.py::TestDataFramePlots::test_plot_with_rot - Failed: DID NOT RAISE <class 'AssertionError'>
=============================================================== 1 failed in 0.46s ===============================================================
```
@TomAugspurger care to look? Thanks.