fix: return empty list from _resolve_crew_skills to prevent redundant per-agent discovery

_resolve_crew_skills returned `resolved or None`, which collapsed an
empty resolved list to None. set_skills treated None as "not provided"
and fell back to re-reading raw crew skill paths per agent, defeating
the purpose of resolving crew skills once.

lib/crewai/src/crewai/crews/utils.py

@@ -75,7 +75,7 @@ def _resolve_crew_skills(crew: Crew) -> list[SkillModel] | None:
                     if item.disclosure_level < INSTRUCTIONS
                     else item
                 )
-    return resolved or None
+    return resolved
 
 
 def setup_agents(