fix dateTimeSchema NaN validation

[jshawl]

This PR fixes an issue where the dateTimeSchema silently errors on NaN values in the start array:

const {error, value} = ics.createEvent({
  start: [NaN, 12, 1]
})
// error === {}
// value === null

This is because:

[1, 2, NaN, 4].indexOf(NaN) === -1

which then throws a TypeError in yup:

TypeError: lazy() functions must return a valid schema

but it was being caught here

ics/src/schema/index.js

Line 137 in 0638524

return {error: Object.assign({}, error), value: undefined}