[fix] 解决可能存在的空指针问题

src/main/java/com/alibaba/excel/context/WriteContextImpl.java

@@ -429,8 +429,10 @@ public class WriteContextImpl implements WriteContext {
             OutputStream outputStream = encryptor.getDataStream(fileSystem);
             opcPackage.save(outputStream);
         } finally {
+            if (opcPackage != null) {
                 opcPackage.close();
             }
+        }
         return fileSystem;
     }
 }