[core] Handle a nullable case in `readResource` from Java

mirai-core-utils/src/jvmBaseMain/kotlin/Resources.kt

@@ -1,5 +1,5 @@
 /*
- * Copyright 2019-2022 Mamoe Technologies and contributors.
+ * Copyright 2019-2023 Mamoe Technologies and contributors.
  *
  * 此源代码的使用受 GNU AFFERO GENERAL PUBLIC LICENSE version 3 许可证的约束, 可以在以下链接找到该许可证.
  * Use of this source code is governed by the GNU AGPLv3 license that can be found through the following link.
@@ -12,5 +12,5 @@ package net.mamoe.mirai.utils
 
 @TestOnly
 public fun readResource(url: String): String =
-    Thread.currentThread().contextClassLoader.getResourceAsStream(url)?.readBytes()?.decodeToString()
+    Thread.currentThread().contextClassLoader?.getResourceAsStream(url)?.readBytes()?.decodeToString()
         ?: error("Could not find resource '$url'")