Undefined array key when formatting numeric cell value

[hivokas]

Thanks for all the work on this package.

I've recently received the following exception in BugSnag:

Looking at the stacktrace, it seems to be internal openspout issue when formatting numeric cell value.

Unfortunately I don't the file to reproduce it, because this request was done by user. I've decided to post this issue anyway in case it can be fixed even without problematic file.