UiTextBox - Allow nulls

I think UiTextBox validation is too strict. passing a v-model with the value of null will trigger a validation error.

data () {
    return {
      name: null
    }
 }

<ui-textbox floating-label label="Name" placeholder="Enter your name" v-model="name"></ui-textbox>

[Vue warn]: Invalid prop: type check failed for prop "value". Expected String, Number, got Null. 
(found in component <ui-textbox> at C:\code\packages\keen-ui\src\UiTextbox.vue)

I believe value.required attribute triggers this. https://github.com/JosephusPaye/Keen-UI/blob/master/src/UiTextbox.vue#L105

[JosephusPaye]

Implemented in https://github.com/JosephusPaye/Keen-UI/commit/f5e63d65d753dd558600f2c8e623b52f4d9f9124.

[JosephusPaye]

Support for null will be added in the next version.