Added @nullable on JShellResult fields

Author: Alathreon

JShellAPI/src/main/java/org/togetherjava/jshellapi/dto/JShellResult.java

@@ -1,13 +1,17 @@
 package org.togetherjava.jshellapi.dto;
 
+import org.springframework.lang.Nullable;
+
 import java.util.List;
 
 public record JShellResult(
         SnippetStatus status,
         SnippetType type,
         int id,
         String source,
+        @Nullable
         String result,
+        @Nullable
         JShellExceptionResult exception,
         boolean stdoutOverflow,
         String stdout,