fix: prompt update in brains management settings tab (#1543)

# Description

#1540 

## Checklist before requesting a review

Please delete options that are not relevant.

- [ ] My code follows the style guidelines of this project
- [ ] I have performed a self-review of my code
- [ ] I have commented hard-to-understand areas
- [ ] I have ideally added tests that prove my fix is effective or that
my feature works
- [ ] New and existing unit tests pass locally with my changes
- [ ] Any dependent changes have been merged

## Screenshots (if appropriate):

frontend/app/brains-management/[brainId]/components/BrainManagementTabs/components/SettingsTab/hooks/useSettingsTab.ts

@@ -258,7 +258,7 @@ export const useSettingsTab = ({ brainId }: UseSettingsTabProps) => {
       }
 
       if (dirtyFields["prompt"]) {
-        if (promptId === "") {
+        if (promptId === "" || promptId === undefined) {
           otherConfigs["prompt_id"] = (
             await createPrompt({
               title: prompt.title,