fix(ui): use null instead of undefined when clearing extra args

Clearing the extra args field set the overlay value to undefined,
which gets dropped during object spread when building the PATCH
payload. The existing extraArgs from the agent config survived the
merge, making it impossible to clear the field. Use null so the
value explicitly overwrites the existing config entry.

Closes #2350

ui/src/components/AgentConfigForm.tsx

@@ -831,7 +831,7 @@ export function AgentConfigForm(props: AgentConfigFormProps) {
                   onCommit={(v) =>
                     isCreate
                       ? set!({ extraArgs: v })
-                      : mark("adapterConfig", "extraArgs", v ? parseCommaArgs(v) : undefined)
+                      : mark("adapterConfig", "extraArgs", v?.trim() ? parseCommaArgs(v) : null)
                   }
                   immediate
                   className={inputClass}