Fix null handling and validation in SamplingRate management

This commit addresses potential runtime exceptions and improves input validation:

- Replace Optional.of() with Optional.ofNullable() in SamplingRateRepository.findById()
  to properly handle cases where no sampling rate is found, preventing NullPointerException
- Add null validation for id parameter in SamplingRateService.deleteSamplingRate()
  to ensure proper error handling before repository operations
- Clean up code formatting in updateSamplingRate() validation block

These changes enhance the robustness of the sampling rate management feature
by preventing NPEs and providing clearer error messages for invalid inputs.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

src/main/java/com/mediamanager/repository/SamplingRateRepository.java

@@ -53,7 +53,7 @@ public class SamplingRateRepository {
         EntityManager em = entityManagerFactory.createEntityManager();
         try{
             SamplingRate samplingRate = em.find(SamplingRate.class, id);
-            return Optional.of(samplingRate);
+            return Optional.ofNullable(samplingRate);
         }finally {
             if (em.isOpen()) em.close();
         }

src/main/java/com/mediamanager/service/samplingrate/SamplingRateService.java

@@ -42,7 +42,8 @@ public class SamplingRateService {
     public Optional<SamplingRate> updateSamplingRate(Integer id, String value) {
     logger.info("Updating sampling rate:{}", value);
     if (value == null || value.trim().isEmpty()) {
-    throw new IllegalArgumentException("Sampling-Rate value cannot be null or empty");}
+        throw new IllegalArgumentException("Sampling-Rate value cannot be null or empty");
+    }
     Optional<SamplingRate> existingSamplingRate = repository.findById(id);
     if(existingSamplingRate.isEmpty()) {
         logger.warn("Sampling rate not found with id:{}", id);
@@ -55,6 +56,9 @@ public class SamplingRateService {
     }
 
     public boolean deleteSamplingRate(Integer id) {
+        if (id == null) {
+            throw new IllegalArgumentException("Sampling rate id cannot be null");
+        }
         logger.info("Deleting sampling rate:{}", id);
         return repository.deleteById(id);
     }