Coerce inspect.getdoc() None to empty string before parsing

`inspect.getdoc()` returns `str | None`, but `docstring_parser.parse()`
requires `str`. Functions without a docstring produced `None`, which
the type checker correctly flagged.

Coerce to `""` at the call site. `docstring_parser.parse("")` returns
an empty docstring whose `.description` and `.params` are already
handled by the surrounding `or ""` fallbacks, so runtime behavior is
unchanged.

src/pipecat/adapters/schemas/direct_function.py

@@ -127,7 +127,7 @@ class BaseDirectFunctionWrapper:
         self.name = self.function.__name__
 
         # Parse docstring for description and parameters
-        docstring = docstring_parser.parse(inspect.getdoc(self.function))
+        docstring = docstring_parser.parse(inspect.getdoc(self.function) or "")
 
         # Get function description
         self.description = (docstring.description or "").strip()