I have set a body::esri:inputMask for a phone field to be "(999) 999-9999".
This field is not set to required.
However, if I don't enter a value for this field I get a warning message that the field input is invalid - presumably because the value is null. And the survey cannot be sent until this field has a valid phone. So, in essence, the body::esri:inputMask is acting as a required setting.
I don't think there should be a warning message for a null value when the body::esri:inputMask is set.