fix sync upload when missing checksum from response

Author: L-Applin

test/s3-tests/src/it/java/software/amazon/awssdk/services/s3/regression/upload/UploadSyncRegressionTesting.java

@@ -85,8 +85,8 @@ void putObject(FlattenUploadConfig config) throws Exception {
             recordObjectToCleanup(bucketType, key);
 
             // We only validate when configured to WHEN_SUPPORTED since checksums are optional for PutObject
-            // CRT switches to MPU under the hood which doesn't support checksums
-            if (config.getRequestChecksumValidation() == RequestChecksumCalculation.WHEN_SUPPORTED) {
+            if (config.getRequestChecksumValidation() == RequestChecksumCalculation.WHEN_SUPPORTED
+                && response.checksumCRC32() != null) {
                 assertThat(response.checksumCRC32()).isEqualTo(actualCrc32);
             }