fix: createdAt npe fixed (#25754)

## Description
> This PR fixes the NPE for anonymous user in userSessionDTO 
#### PR fixes following issue(s)
Fixes #25753 

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#### Type of change
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- Bug fix (non-breaking change which fixes an issue)

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## Testing
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#### How Has This Been Tested?
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- [ ] Manual
- [ ] Jest
- [ ] Cypress
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## Checklist:
#### Dev activity
- [x] My code follows the style guidelines of this project
- [x] I have performed a self-review of my own code
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app/server/appsmith-server/src/main/java/com/appsmith/server/dtos/UserSessionDTO.java

@@ -73,7 +73,10 @@ public class UserSessionDTO {
         session.email = user.getEmail();
         session.hashedEmail = user.getHashedEmail();
         session.name = user.getName();
-        session.createdAt = user.getCreatedAt().getEpochSecond();
+        // user.getCreatedAt() is null for anonymous user
+        if (user.getCreatedAt() != null) {
+            session.createdAt = user.getCreatedAt().getEpochSecond();
+        }
         session.source = user.getSource();
         session.state = user.getState();
         session.isEnabled = user.isEnabled();
@@ -110,7 +113,10 @@ public class UserSessionDTO {
         user.setEmail(email);
         user.setHashedEmail(hashedEmail);
         user.setName(name);
-        user.setCreatedAt(Instant.ofEpochSecond(createdAt));
+        // createdAt is null for anonymous user
+        if (createdAt != null) {
+            user.setCreatedAt(Instant.ofEpochSecond(createdAt));
+        }
         user.setSource(source);
         user.setState(state);
         user.setIsEnabled(isEnabled);