[core] Throw ISE when "user.dir" is not available, instead of throwing NPE

mirai-core-utils/src/jvmBaseMain/kotlin/MiraiFile.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.
@@ -62,7 +62,10 @@ public actual interface MiraiFile {
         }
 
         public actual fun getWorkingDir(): MiraiFile {
-            return create(System.getProperty("user.dir"))
+            return create(
+                System.getProperty("user.dir")
+                    ?: throw IllegalStateException("System property 'user.dir' is not available")
+            )
         }
     }
 }