Fix: some agent bug (#14829)

### What problem does this PR solve?

fix: 
update null checks to use 'is None' for better clarity
replace RAGFlowSelect with SelectWithSearch in DebugContent
add max height and overflow to DialogContent in ParameterDialog
 remove unused types from DataOperationsForm

### Type of change

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

agent/component/message.py

@@ -161,7 +161,7 @@ class Message(ComponentBase):
             if k in kwargs:
                 continue
             v = v["value"]
-            if not v:
+            if v is None:
                 v = ""
             ans = ""
             if isinstance(v, partial):

agent/component/string_transform.py

@@ -105,7 +105,7 @@ class StringTransform(Message, ABC):
                 pass
 
         for k,v in kwargs.items():
-            if not v:
+            if v is None:
                 v = ""
             script = re.sub(k, lambda match: v, script)