Merge pull request #750 from 3863224zql/master

[fix] 解决潜在的空指针问题

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

@@ -429,7 +429,9 @@ public class WriteContextImpl implements WriteContext {
             OutputStream outputStream = encryptor.getDataStream(fileSystem);
             opcPackage.save(outputStream);
         } finally {
-            opcPackage.close();
+            if (opcPackage != null) {
+                opcPackage.close();
+            }
         }
         return fileSystem;
     }