Make sure splitlines is not called on None (#161)

Small change that avoids that `splitlines` is called on a `None` object;
this may currently happen if `prop.fget.__doc__` exists and is `None`,
thus causing an exception (`'NoneType' object has no attribute
'splitlines'`).

Co-authored-by: Tosco, Paolo <[email protected]>

pybind11_stubgen/parser/mixins/parse.py

@@ -502,7 +502,7 @@ def handle_property(self, path: QualifiedName, prop: Any) -> Property | None:
                 result.doc == result.getter.doc
                 or result.doc
                 == self._strip_empty_lines(
-                    getattr(prop.fget, "__doc__", "").splitlines()
+                    (getattr(prop.fget, "__doc__", "") or "").splitlines()
                 )
             )
         ):