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>

tests/config_none_context/agents.yaml

@@ -0,0 +1,5 @@
+test_agent:
+  role: Test Agent
+  goal: Test goal
+  backstory: Test backstory
+  verbose: true

tests/config_none_context/tasks.yaml

@@ -0,0 +1,11 @@
+task_with_none_context:
+  description: A test task with None context
+  expected_output: Some output
+  agent: test_agent
+  context: None
+
+task_with_valid_context:
+  description: A test task with valid context
+  expected_output: Some output
+  agent: test_agent
+  context: [task_with_none_context]