Fixes Bug in InMemoryFileRepository (#738)

Co-authored-by: Aaron Zielstorff <aaron.zi@web.de>

Author: FriedJannik

basyx.common/basyx.filerepository-backend-inmemory/src/main/java/org/eclipse/digitaltwin/basyx/core/filerepository/InMemoryFileRepository.java

@@ -92,6 +92,7 @@ public void delete(String fileId) throws FileDoesNotExistException {
 
 	@Override
 	public boolean exists(String fileId) {
+		if(fileId == null) return false;
 
 		if (fileId.isBlank() || !isFilePathValid(fileId))
 			return false;