Bug: Clicking the validate button (Ok) in range-mode without picking both start and end date triggers an error

[kfirba]

@mathieustan it's me again.

A bug I found is when you open the datepicker in range mode with the validation buttons option turned on and click "Ok" without picking any date or not picking the end date, an error is triggered:

vue.common.dev.js?4650:1893 TypeError: Cannot read property 'format' of undefined
    at formatDateToSend (vue-datepicker.esm.js?bd0e:674)
    at VueComponent.validateDate (vue-datepicker.esm.js?bd0e:4344)
    at invokeWithErrorHandling (vue.common.dev.js?4650:1859)
    at Proxy.invoker (vue.common.dev.js?4650:2184)
    at invokeWithErrorHandling (vue.common.dev.js?4650:1859)
    at Proxy.Vue.$emit (vue.common.dev.js?4650:3879)
    at Proxy.Vue.<computed> (backend.js:1781)
    at validate (vue-datepicker.esm.js?bd0e:3945)
    at invokeWithErrorHandling (vue.common.dev.js?4650:1859)
    at Proxy.invoker (vue.common.dev.js?4650:2184)

[mathieustan]

Should be fixed in latest release 0.1.9