fix: shaper exception when retriever return 0 docs. (#4929)

* When retriever retrieves 0 documents from the documentStore,
  shaper will raise an exception.

Signed-off-by: root <root@aia-opa-clx-4017.jf.intel.com>
Co-authored-by: root <root@aia-opa-clx-4017.jf.intel.com>

haystack/nodes/other/shaper.py

@@ -740,7 +740,7 @@ def run(  # type: ignore
         if labels and "labels" not in invocation_context.keys():
             invocation_context["labels"] = labels
 
-        if documents and "documents" not in invocation_context.keys():
+        if documents != None and "documents" not in invocation_context.keys():
             invocation_context["documents"] = documents
 
         if meta and "meta" not in invocation_context.keys():