Setting up required fields that can be empty with Zend\InputFilter – Rob Allen's DevNotes

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When you create an input filter entry that has the required element set to true, then by default, the field is set up so that it must have a value in it. If you want the field to be required, but also an empty value is okay, then add the allow_empty element to your definition:

In this definition, we have a 'notes' element that must exist, but can be an empty string.

That's all there is to it.