fix(inputs): reset value to null instead of undefined

[KaelWD]

Description

This fixes some inconsistencies when resetting input fields.

• v-text-field would reset to null when clear was clicked, but undefined in a form.
• v-select and variants, and v-file-input would reset to undefined or [] if multiple.
• v-autocomplete and v-combobox would reset search-input to null, unless clear was clicked then it would be undefined instead

With this change these will all be null or [], allowing values to be sent as JSON without having to check for undefined first.

This is technically a breaking change if someone was relying on form.reset() to reset inputs to undefined but this is unlikely and can easily be worked around by setting the values manually afterwards.
v-select also now re-selects an item with value: null if the clear button is clicked (#4431, unit test). Again, unlikely to actually be a problem (either intended behaviour or not using both clearable and a null item) and easily worked around by implementing the clear button yourself with append-icon and @click:clear.

The consistency is worth these low-impact changes in my opinion.

Some sort of clear-value prop (#4144, #7441) would be even better, but the PR was abandoned and apparently had problems with non-falsy values.

fixes #7429

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Improvement/refactoring (non-breaking change that doesn't add any features but makes things better)

---

Untested outside of existing unit tests btw, might be scuffed

[sindilevich]

Hello, this is a great PR that tremendously streamlines handling inputs' data!
I thought to highlight, however, that some traces of undefined are still left unaltered. I can't tell whether they'd matter at all, but thought it'd better to point them out:

• vuetify/packages/vuetify/src/components/VAutocomplete/VAutocomplete.ts

  Line 54 in 4a1bf59

  default: undefined,

• vuetify/packages/vuetify/src/components/VAutocomplete/VAutocomplete.ts

  Line 99 in 4a1bf59

  get (): string | undefined {

• vuetify/packages/vuetify/src/components/VAutocomplete/VAutocomplete.ts

  Line 180 in 4a1bf59

  this.lazySearch = undefined

• vuetify/packages/vuetify/src/components/VAutocomplete/VAutocomplete.ts

  Line 288 in 4a1bf59

  this.setValue(this.multiple ? [] : undefined)

• vuetify/packages/vuetify/src/components/VFileInput/VFileInput.ts

  Lines 69 to 70 in 4a1bf59

  	value: {
  	default: undefined,

• vuetify/packages/vuetify/src/components/VSelect/VSelect.ts

  Lines 122 to 124 in 4a1bf59

  	lazyValue: this.value !== undefined
  	? this.value
  	: this.multiple ? [] : undefined,

Hope it helps, and thank you.

[KaelWD]

Good catch, thanks.

[johnleider]

It's outside of this issue's scope but I think we should explore exposing the reset process so the user could set it to whatever they want.

[sindilevich]

It's outside of this issue's scope but I think we should explore exposing the reset process so the user could set it to whatever they want.

Letting user a way to control the reset value - is a great idea! Hope someone could come up with an appropriate PR.