fix: handle empty documents list in KnowledgeStorage.save() and asave()

Fixes #4277

When KnowledgeStorage.save() or asave() is called with an empty documents
list, the method now returns early instead of propagating a low-level
ValueError from ChromaDB's upsert operation.

This is a valid edge case that can occur in real-world workflows (e.g.,
after filtering, retrieval failures, or conditional logic), and should
be handled gracefully as a no-op.

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

lib/crewai/src/crewai/knowledge/storage/knowledge_storage.py

@@ -99,6 +99,9 @@ class KnowledgeStorage(BaseKnowledgeStorage):
             )
 
     def save(self, documents: list[str]) -> None:
+        if not documents:
+            return
+
         try:
             client = self._get_client()
             collection_name = (
@@ -177,6 +180,9 @@ class KnowledgeStorage(BaseKnowledgeStorage):
         Args:
             documents: List of document strings to save.
         """
+        if not documents:
+            return
+
         try:
             client = self._get_client()
             collection_name = (