Don't add empty desc nodes

Empty desc nodes break LaTeX markup and are not permitted according to the [Sphinx documentation](http://sphinx-doc.org/extdev/nodes.html#sphinx.addnodes.desc):

> It (desc node) contains one or more desc_signature and a desc_content.

This partially addresses issue https://github.com/michaeljones/breathe/issues/127.

Also the node insertion code is slightly simplified (`try-except` block is not needed):

```python
try:
  # As "user-defined" can repeat
  section_nodelists[kind] += [node]
except KeyError:
  section_nodelists[kind] = [node]
```

changed to

```python
# As "user-defined" can repeat
nodes = section_nodelists.setdefault(kind, [])
nodes += [node]
```