Avoid ArrayIndexOutOfBoundsException when getting cell style w/ no cellStyleXfs

I ran into this issue via org.apache.poi.ss.usermodel.DateUtil.isCellDateFormatted.

Essentially, the issue is that I'm working with xlsx spreadsheets that do not
contain a <cellStyleXfs/> section.  The /cellXfs xf.xfId/ attributes are
set to "0".  Instead of blowing up, I'd like poi to recognize that there
is no such named style and proceed normally.