add null checker to sourceRow so it can be skipped if it's null

[xjlin0]

When copying the sheet in the memory, if the row is null, currently it will cause NPE. So just a null checker to protect it, similar to how it skips null columns as of now. @zmau

2020-04-23 09:49:07.390 [,myservice,,] ERROR https-jsse-nio-9420-exec-2 [dispatcherServlet] - Servlet.service() for servlet [dispatcherServlet] in context with path [] threw exception [Request processing failed; nested exception is java.lang.NullPointerException] with root cause
java.lang.NullPointerException: null
        at org.apache.poi.ss.usermodel.RangeCopier.copyRange(RangeCopier.java:89)
        at org.apache.poi.ss.usermodel.RangeCopier.copyRange(RangeCopier.java:68)

[pjfanning]

Would you be able to provide a test xlsx file and a test case that reproduces the issue? We like to have regression tests.

[centic9]

Sorry, I was too fast, I could quickly reproduce it and fix it as part of some other improvement to the tests.

[xjlin0]

it's ok, as long as the null row can now be skipped and will be release in the next version, I will be happy.