fix: Allow text fields to have an initial value of 0

Every time maskText was called, it had an attempted null check: || ''
This check meant that any falsy values would get converted into
an empty string.
I removed all these checks and replaced them with a proper one
in mask.js@maskText()

Fixes vuetifyjs#2136

src/components/VTextField/VTextField.js

@@ -182,7 +182,7 @@ export default {
           autofocus: this.autofocus,
           disabled: this.disabled,
           required: this.required,
-          value: this.maskText(this.lazyValue || '')
+          value: this.maskText(this.lazyValue)
         },
         attrs: {
           ...this.$attrs,

src/components/VTextField/VTextField.spec.js

@@ -331,4 +331,12 @@ test('VTextField.js', ({ mount }) => {
 
     expect(wrapper.html()).toMatchSnapshot()
   })
+
+  it('should display the number 0', async () => {
+    const wrapper = mount(VTextField, {
+      propsData: { value: 0 }
+    })
+
+    expect(wrapper.vm.$refs.input.value).toBe('0')
+  })
 })

src/mixins/maskable.js

@@ -55,8 +55,8 @@ export default {
     mask () {
       if (!this.$refs.input) return
 
-      const oldText = this.$refs.input.value || ''
-      const newText = this.maskText(this.lazyValue || '')
+      const oldText = this.$refs.input.value
+      const newText = this.maskText(this.lazyValue)
       let position = 0
       let selection = this.selection
 

src/util/mask.js

@@ -90,6 +90,7 @@ const maskValidates = (mask, char) => {
  * @return {String}
  */
 export const maskText = (text, masked, dontFillMaskBlanks) => {
+  if (text == null) return ''
   text = String(text)
   if (!masked.length || !text.length) return text
   if (!Array.isArray(masked)) masked = masked.split('')