Sphinx 7.4.7 compat

[mweinelt]

Hi! We're running the test suite for breathe 4.35.0 as part of integrating breathe into nixpkgs and notice test failures after upgrading sphinx from 7.3.7 to 7.4.7. I've confirmed this is also the case on 62695c7.

The complete log is here: https://gist.github.com/mweinelt/1e6a698cf1c5a24ac1467422e996a035

breathe> FAILED tests/test_renderer.py::test_render_func - TypeError: expected string or bytes-like object, got 'NoneType'
breathe> FAILED tests/test_renderer.py::test_render_typedef - TypeError: expected string or bytes-like object, got 'NoneType'
breathe> FAILED tests/test_renderer.py::test_render_c_typedef - TypeError: expected string or bytes-like object, got 'NoneType'
breathe> FAILED tests/test_renderer.py::test_render_c_function_typedef - TypeError: expected string or bytes-like object, got 'NoneType'
breathe> FAILED tests/test_renderer.py::test_render_using_alias - TypeError: expected string or bytes-like object, got 'NoneType'
breathe> FAILED tests/test_renderer.py::test_render_const_func - TypeError: expected string or bytes-like object, got 'NoneType'
breathe> FAILED tests/test_renderer.py::test_render_lvalue_func - TypeError: expected string or bytes-like object, got 'NoneType'
breathe> FAILED tests/test_renderer.py::test_render_rvalue_func - TypeError: expected string or bytes-like object, got 'NoneType'
breathe> FAILED tests/test_renderer.py::test_render_const_lvalue_func - TypeError: expected string or bytes-like object, got 'NoneType'
breathe> FAILED tests/test_renderer.py::test_render_const_rvalue_func - TypeError: expected string or bytes-like object, got 'NoneType'
breathe> FAILED tests/test_renderer.py::test_render_variable_initializer - TypeError: expected string or bytes-like object, got 'NoneType'
breathe> FAILED tests/test_renderer.py::test_render_define_initializer - TypeError: expected string or bytes-like object, got 'NoneType'
breathe> FAILED tests/test_renderer.py::test_render_define_no_initializer - TypeError: expected string or bytes-like object, got 'NoneType'