Warn if the value of a SelectOne or SelectMultiple option is blank

[florianm]

Problem

If a form definition has multiple languages and a selectOne has missing human-readable labels in all languages for at least one selectOne option, the form is still valid in Central and Collect, but throws an error in ruODK::form_schema_ext, which assumes that all options have values.

TODO: verify whether this applies to SelectMultiple (assume yes) and single translation. Build and test example forms with missing values.

Error message

"Error in xml_nodeset(NextMethod()): Expecting an external pointer: [type=NULL]"

Forum post

https://forum.getodk.org/t/ruodk-error-for-form-schema-ext-expecting-an-external-pointer-type-null/38276

Approach

ODK Build could warn if a form has a blank SelectOne or SelectMultiple option label.

Why so: Is there any reason a form should have no user visible label for an option?
Why not: the XForm and XLSForms specs seem to allow this, seeing that such a form works both in Central and Collect.