OZG-7426 do send method synchronized

ozgcloud-common-lib/src/main/java/de/ozgcloud/common/binaryfile/GrpcFileUploadUtils.java

@@ -112,8 +112,9 @@ public class GrpcFileUploadUtils {
 			closeStreams();
 		}
 
-		void sendNext() {
+		synchronized void sendNext() {
 			if (done.get()) {
+				LOG.debug("Sending is finished (fileSenderId = {})", fileSenderId);
 				return;
 			}
 			waitForObserver();
@@ -121,7 +122,7 @@ public class GrpcFileUploadUtils {
 			while (!done.get() && isReady()) {
 				sendNextChunk();
 			}
-			LOG.debug("Finished or waiting to become ready. (fileSenderId = {})", fileSenderId);
+			LOG.debug("Finished ({}) or waiting to become ready. (fileSenderId = {})", done.get(), fileSenderId);
 		}
 
 		boolean isReady() {