Allow handling empty string value in xls formula parsing

[7Towers]

#343
Simple fix to handle empty string values when parsing XLS formulas. Based on MS recommendations. Fixes the issue I've faced in my projects.

[dimastbk]

Test file - issue_343.xls (put ="" in A1).

[tafia]

Yes please add a test. Thanks!

[dimastbk]

I think, 0x03 should return DataType::String("".to_string()), because empty cell != zero-lenght string cell. See for example xlrd and sheetjs.

[jqnatividad]

Hope you don't mind @7Towers , but I went ahead and implemented an alternate PR using @dimastbk 's recommendation to return an empty string, using the test file he provided.

Hopefully, when this gets merged, the next calamine release will be published by @tafia 😉 🤞

[7Towers]

Great job @jqnatividad. Your PR is better for the project.