fix mutiple retrieval component content (#1997)

### What problem does this PR solve?

### Type of change

- [x] Bug Fix (non-breaking change which fixes an issue)

agent/component/base.py

@@ -448,6 +448,8 @@ class ComponentBase(ABC):
             if self.component_name.lower() == "generate" and self.get_component_name(u) == "retrieval":
                 o = self._canvas.get_component(u)["obj"].output(allow_partial=False)[1]
                 if o is not None:
+                    if not "".join(o["content"]):
+                        continue
                     upstream_outs.append(o)
                     continue
             if u not in self._canvas.get_component(self._id)["upstream"]: continue