fix: Don't allow None for a property's docstring

Author: pawamoy

src/pytkdocs/loader.py

@@ -387,7 +387,7 @@ def get_function_documentation(self, node: ObjectNode) -> Function:
             name=node.name,
             path=node.dotted_path,
             file_path=node.file_path,
-            docstring=inspect.getdoc(function),
+            docstring=inspect.getdoc(function) or "",
             signature=signature,
             source=source,
         )
@@ -425,7 +425,7 @@ def get_property_documentation(self, node: ObjectNode) -> Attribute:
             name=node.name,
             path=path,
             file_path=node.file_path,
-            docstring=inspect.getdoc(prop.fget),
+            docstring=inspect.getdoc(prop.fget) or "",
             attr_type=attr_type,
             properties=properties,
             source=source,
@@ -555,7 +555,7 @@ def get_method_documentation(self, node: ObjectNode, properties: Optional[List[s
             name=node.name,
             path=path,
             file_path=node.file_path,
-            docstring=inspect.getdoc(method),
+            docstring=inspect.getdoc(method) or "",
             signature=inspect.signature(method),
             properties=properties or [],
             source=source,