Handle the format '?' in number formatting

[kurtinge]

Handle the format '?' in number formatting

Description

When excelize tries to read a cell where the formatCode in the style is "?" it will return an empty string instead of the number.

Motivation and Context

This handles that the formatCode "?" is valid and returns a the number correctly

How Has This Been Tested

This has been tested in the unit test and with an actual file.

Types of changes

• Docs change / refactoring / dependency upgrade
• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.