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.

pyrightconfig.json

@@ -2,11 +2,16 @@
   "typeCheckingMode": "basic",
   "pythonVersion": "3.11",
   "pythonPlatform": "All",
-  "include": ["scripts", "src/pipecat"],
-  "exclude": ["**/*_pb2.py", "**/__pycache__"],
+  "include": [
+    "scripts",
+    "src/pipecat"
+  ],
+  "exclude": [
+    "**/*_pb2.py",
+    "**/__pycache__"
+  ],
   "ignore": [
     "tests",
-    "src/pipecat/adapters/schemas/direct_function.py",
     "src/pipecat/adapters/services/anthropic_adapter.py",
     "src/pipecat/adapters/services/aws_nova_sonic_adapter.py",
     "src/pipecat/adapters/services/bedrock_adapter.py",