Fix task configuration with None context parameter in YAML

Fixes #3696

When context: None is specified in YAML, yaml.safe_load() converts it to
the string 'None' instead of Python's None object. The code was attempting
to iterate over this string character by character, causing KeyError: 'N'
when trying to look up task names.

Changes:
- Added isinstance(context_list, list) check in crew_base.py before
  iterating context_list to handle YAML's conversion of None to string
- Added test case test_task_with_none_context_from_yaml to verify tasks
  can be configured with context: None without errors
- Added test YAML configurations in tests/config_none_context/ to
  reproduce and verify the fix

The fix ensures that only actual list values are processed, allowing
None and other non-list values to pass through without causing errors.

Co-Authored-By: João <joao@crewai.com>

src/crewai/project/crew_base.py

@@ -260,9 +260,10 @@ def CrewBase(cls: T) -> T:  # noqa: N802
             output_pydantic_functions: dict[str, Callable],
         ) -> None:
             if context_list := task_info.get("context"):
-                self.tasks_config[task_name]["context"] = [
-                    tasks[context_task_name]() for context_task_name in context_list
-                ]
+                if isinstance(context_list, list):
+                    self.tasks_config[task_name]["context"] = [
+                        tasks[context_task_name]() for context_task_name in context_list
+                    ]
 
             if tools := task_info.get("tools"):
                 self.tasks_config[task_name]["tools"] = [