fixed null conversation for strToUpper

[brainfoolong]

fixed in the same way as it already has been done for strToLower

This is:

- [x] a bugfix

Checklist:

• Changes are covered by unit tests
• Code style is respected
• Commit message explains why the change is made (see https://github.com/erlang/otp/wiki/Writing-good-commit-messages)
• CHANGELOG.md contains a short summary of the change
• Documentation is updated as necessary

Why this change is needed?

Just a fix applied to strToUpper in the same way as it has already been done for strToLower.

[oleibman]

Might you be able to add a formal test which fails with the existing code but succeeds with your change?

[brainfoolong]

Unfortunetely i don't have an example. The error happens for me, when i load an existing excel file (that was creating by unknown software) that i can't publish here. I was unable to reproduce it in a way that i can post here. As soon as i manipulate the excel and re-save it without sensible information, the error is gone.

I tried, but without success. Also i don't had time to invest more time.

Sorry.