fix: handle None state values in skill_toolset after session rewind

Session rewind sets state keys to None as deletion markers. dict.get()
returns None (not the default) when the key exists with value None,
causing list(None) to raise TypeError.

Change `state.get(key, [])` to `state.get(key) or []` at both call
sites so that explicit None values fall back to an empty list.

Github-Issue: #5193
Reported-by: SAMFVH

Author: enjoykumawat

src/google/adk/tools/skill_toolset.py

@@ -152,7 +152,7 @@ async def run_async(
     agent_name = tool_context.agent_name
     state_key = f"_adk_activated_skill_{agent_name}"
 
-    activated_skills = list(tool_context.state.get(state_key, []))
+    activated_skills = list(tool_context.state.get(state_key) or [])
     if skill_name not in activated_skills:
       activated_skills.append(skill_name)
       tool_context.state[state_key] = activated_skills
@@ -791,7 +791,7 @@ async def _resolve_additional_tools_from_state(
 
     agent_name = readonly_context.agent_name
     state_key = f"_adk_activated_skill_{agent_name}"
-    activated_skills = readonly_context.state.get(state_key, [])
+    activated_skills = readonly_context.state.get(state_key) or []
 
     if not activated_skills:
       return []