`validateMany` should return `null` when all validations pass

validateMany currently returns an array of results ([null]), even if all subvalidations successfully pass. null should be returned directly to be consistent with validate and allow nesting of validate and validateMany calls.

validate({ 
  foo: [isRequired] 
})({ 
  foo: 'bar' 
}); // === null

validateMany({ 
  foo: [isRequired] 
})([{ 
  foo: 'bar' 
}]) // === [null], expected to be null

validate({
  arr: [
    validateMany({ 
      foo: [isRequired] 
    })
  ]
})({
  arr: [{ 
    foo: 'bar' 
  }]
}) // === {arr: [null]}, expected to be null

Of course when at least one validation fails, the structure should be kept as is to know which index is wrong.

Proposed fix:

const fixedValidateMany = validations => o(when(all(isNil), always(null)), validateMany(validations));

Start with the validateMany entry point and compare its successful-result behavior with validate. Add coverage for all validations passing, including nesting validateMany inside validate, while preserving the indexed structure when any validation fails; done means the all-pass result is null.