CXSmiles writer does not use default conformer ID

[greglandrum]

Describe the bug
Here's the reproducible;

In [3]: m = Chem.MolFromSmiles('CC |(-0.75,0,;0.75,0,)|')

In [4]: m.GetConformer().SetId(5)

In [5]: Chem.MolToCXSmiles(m)
---------------------------------------------------------------------------
ValueError                                Traceback (most recent call last)
Cell In[5], line 1
----> 1 Chem.MolToCXSmiles(m)

ValueError: Bad Conformer Id

Configuration (please complete the following information):

• RDKit version: master and releases
• OS: all

Additional context

This is the root cause of #7409