fix(presence): fix doc and add option to allow empty string

[GreatWizard]

Problem

The current ignoreBlank option said it treats an empty string as not present.
But as you can see in the test, this exact use case throw an error.

Current solution

• Add test about the failing use case
• Add a new allowEmpty option to handle that specific use case and not break the current integration
• Update the JSDoc accordingly