Having title_quantiles=None when quantiles is passed without having exactly 3 elements raises a ValueError

Hi,

The behaviour of the title_quantiles argument that was introduced in v2.2.2 is responsible for a crash when using the following set of arguments:
show_titles=True, title_quantiles=None
and a list for quantiles that has a number of elements other than 3. In this case, according to what is written in the documentation, the value for title_quantiles is set as what is passed for quantiles, and a ValueError is raised.

In particular, this is an issue for compatibility with codes that were running with corner versions older than v2.2.2.

The error can be reproduced by slightly modifying the code from the Getting started tutorial (https://corner.readthedocs.io/en/latest/pages/quickstart/):

import corner
import numpy as np

ndim, nsamples = 2, 10000
np.random.seed(42)
samples = np.random.randn(ndim * nsamples).reshape([nsamples, ndim])
figure = corner.corner(samples, quantiles=[0.16, 0.84], 
                       show_titles=True, title_quantiles=None)

Running this code, the following error is raised:
ValueError: 'title_quantiles' must contain exactly three values; pass a length-3 list or array using the 'title_quantiles' argument

Start at the corner.corner entry point and reproduce the issue with the modified Getting started example using quantiles=[0.16, 0.84], show_titles=True, and title_quantiles=None. Trace how these arguments are validated and confirm that the compatibility case completes without the reported ValueError.