fix: added ff fallback if cs is down (#27187)

## Description
> This PR returns only the local flags if the CS is down and there are
no cached flags.

#### PR fixes following issue(s)
Fixes #25977 

#### Type of change
- Bug fix 

## Testing
#### How Has This Been Tested?
> Please describe the tests that you ran to verify your changes. Also
list any relevant details for your test configuration.
> Delete anything that is not relevant
- [x] Manual
- [ ] JUnit
- [ ] Jest
- [ ] Cypress
>
>
#### Test Plan
> Add Testsmith test cases links that relate to this PR
>
>
#### Issues raised during DP testing
> Link issues raised during DP testing for better visiblity and tracking
(copy link from comments dropped on this PR)
>
>
>
## Checklist:
#### Dev activity
- [x] My code follows the style guidelines of this project
- [x] I have performed a self-review of my own code
- [x] I have commented my code, particularly in hard-to-understand areas
- [ ] I have made corresponding changes to the documentation
- [x] My changes generate no new warnings
- [ ] I have added tests that prove my fix is effective or that my
feature works
- [x] New and existing unit tests pass locally with my changes
- [ ] PR is being merged under a feature flag


#### QA activity:
- [ ] [Speedbreak
features](https://github.com/appsmithorg/TestSmith/wiki/Guidelines-for-test-plans#speedbreakers-)
have been covered
- [ ] Test plan covers all impacted features and [areas of
interest](https://github.com/appsmithorg/TestSmith/wiki/Guidelines-for-test-plans#areas-of-interest-)
- [ ] Test plan has been peer reviewed by project stakeholders and other
QA members
- [ ] Manually tested functionality on DP
- [ ] We had an implementation alignment call with stakeholders post QA
Round 2
- [ ] Cypress test cases have been added and approved by SDET/manual QA
- [ ] Added `Test Plan Approved` label after Cypress tests were reviewed
- [ ] Added `Test Plan Approved` label after JUnit tests were reviewed

app/server/appsmith-server/src/main/java/com/appsmith/server/services/ce/CacheableFeatureFlagHelperCEImpl.java

@@ -22,11 +22,13 @@ import com.appsmith.server.solutions.ReleaseNotesService;
 import com.appsmith.util.WebClientUtils;
 import lombok.RequiredArgsConstructor;
 import lombok.extern.slf4j.Slf4j;
+import org.apache.commons.lang3.ObjectUtils;
 import org.springframework.core.ParameterizedTypeReference;
 import org.springframework.web.reactive.function.BodyInserters;
 import reactor.core.publisher.Mono;
 
 import java.time.Instant;
+import java.time.temporal.ChronoUnit;
 import java.util.HashMap;
 import java.util.Map;
 import java.util.Set;
@@ -51,6 +53,11 @@ public class CacheableFeatureFlagHelperCEImpl implements CacheableFeatureFlagHel
             CachedFlags cachedFlags = new CachedFlags();
             cachedFlags.setRefreshedAt(Instant.now());
             cachedFlags.setFlags(flags);
+            // if cs is down, returning the empty flags, so the request doesn't error out.
+            // setting the refreshed at to an older time, so that the next call reaches to CS
+            if (flags.isEmpty()) {
+                cachedFlags.setRefreshedAt(Instant.now().minus(1, ChronoUnit.DAYS));
+            }
             return Mono.just(cachedFlags);
         });
     }
@@ -105,7 +112,7 @@ public class CacheableFeatureFlagHelperCEImpl implements CacheableFeatureFlagHel
                     return this.getRemoteFeatureFlagsByIdentity(new FeatureFlagIdentityTraits(
                             objects.getT1(), tenantId, Set.of(userIdentifier), objects.getT3()));
                 })
-                .map(newValue -> newValue.get(userIdentifier));
+                .map(newValue -> ObjectUtils.defaultIfNull(newValue.get(userIdentifier), Map.of()));
     }
 
     /**