Fix: handle waiting tasks when upstream is switch/categorize/relevant and normal path fails (#6874)

### What problem does this PR solve?

Fix the issue where waiting tasks couldn't be processed when upstream
components were "switch", "categorize", or "relevant" and the normal
processing path couldn't continue.

### Type of change

- [x] Bug Fix (non-breaking change which fixes an issue)
- [ ] New Feature (non-breaking change which adds functionality)
- [ ] Documentation Update
- [ ] Refactoring
- [ ] Performance Improvement
- [ ] Other (please describe):

agent/canvas.py

@@ -252,15 +252,15 @@ class Canvas:
             if loop:
                 raise OverflowError(f"Too much loops: {loop}")
 
+            downstream = []
             if cpn["obj"].component_name.lower() in ["switch", "categorize", "relevant"]:
                 switch_out = cpn["obj"].output()[1].iloc[0, 0]
                 assert switch_out in self.components, \
                     "{}'s output: {} not valid.".format(cpn_id, switch_out)
-                for m in prepare2run([switch_out]):
-                    yield {"content": m, "running_status": True}
-                continue
-
+                downstream = [switch_out]
+            else:
                 downstream = cpn["downstream"]
+
             if not downstream and cpn.get("parent_id"):
                 pid = cpn["parent_id"]
                 _, o = cpn["obj"].output(allow_partial=False)