Fixed nulls sorting (#1347)

Author: germanosin

api/src/main/java/io/kafbat/ui/controller/SchemasController.java

@@ -241,7 +241,7 @@ public Mono<ResponseEntity<SchemaSubjectsResponseDTO>> getSchemas(String cluster
 
           List<String> subjectsToRetrieve;
           boolean paginate = true;
-          var schemaComparator = getComparatorForSchema(orderBy);
+          var schemaComparator = Comparator.nullsFirst(getComparatorForSchema(orderBy));
           final Comparator<SubjectWithCompatibilityLevel> comparator =
               sortOrder == null || !sortOrder.equals(SortOrderDTO.DESC)
                   ? schemaComparator : schemaComparator.reversed();