fix: cannot use empty string in defaultValue on text-like fields (#6842)

## Description

This changes allows empty strings (`''`) to be used as defaultValue for fields of types:
`'text'`; `'textarea'`; `'email'`; `'code'`. This can be useful when you
want to ensure the value is always a `string` instead of
`null`/`undefined`.


- [x] I have read and understand the
[CONTRIBUTING.md](https://github.com/payloadcms/payload/blob/main/CONTRIBUTING.md)
document in this repository.

## Type of change

- [x] New feature (non-breaking change which adds functionality)

## Checklist:

- [x] I have added tests that prove my fix is effective or that my
feature works
- [x] Existing test suite passes locally with my changes

packages/payload/src/fields/config/schema.ts

@@ -85,7 +85,7 @@ export const text = baseField.keys({
       .try(joi.object().pattern(joi.string(), [joi.string()]), joi.string()),
     rtl: joi.boolean(),
   }),
-  defaultValue: joi.alternatives().try(joi.string(), joi.func()),
+  defaultValue: joi.alternatives().try(joi.string().allow(''), joi.func()),
   hasMany: joi.boolean().default(false),
   maxLength: joi.number(),
   maxRows: joi.number().when('hasMany', { is: joi.not(true), then: joi.forbidden() }),
@@ -141,7 +141,7 @@ export const textarea = baseField.keys({
     rows: joi.number(),
     rtl: joi.boolean(),
   }),
-  defaultValue: joi.alternatives().try(joi.string(), joi.func()),
+  defaultValue: joi.alternatives().try(joi.string().allow(''), joi.func()),
   maxLength: joi.number(),
   minLength: joi.number(),
 })
@@ -159,7 +159,7 @@ export const email = baseField.keys({
     }),
     placeholder: joi.string(),
   }),
-  defaultValue: joi.alternatives().try(joi.string(), joi.func()),
+  defaultValue: joi.alternatives().try(joi.string().allow(''), joi.func()),
   maxLength: joi.number(),
   minLength: joi.number(),
 })
@@ -175,7 +175,7 @@ export const code = baseField.keys({
     editorOptions: joi.object().unknown(), // Editor['options'] @monaco-editor/react
     language: joi.string(),
   }),
-  defaultValue: joi.alternatives().try(joi.string(), joi.func()),
+  defaultValue: joi.alternatives().try(joi.string().allow(''), joi.func()),
 })
 
 export const json = baseField.keys({

test/fields/collections/Text/index.ts

@@ -40,6 +40,16 @@ const TextFields: CollectionConfig = {
       },
       type: 'text',
     },
+    {
+      name: 'defaultString',
+      defaultValue: defaultText,
+      type: 'text',
+    },
+    {
+      name: 'defaultEmptyString',
+      defaultValue: '',
+      type: 'text',
+    },
     {
       name: 'defaultFunction',
       defaultValue: () => defaultText,

test/fields/int.spec.ts

@@ -82,10 +82,15 @@ describe('Fields', () => {
 
     it('creates with default values', () => {
       expect(doc.text).toEqual(text)
+      expect(doc.defaultString).toEqual(defaultText)
       expect(doc.defaultFunction).toEqual(defaultText)
       expect(doc.defaultAsync).toEqual(defaultText)
     })
 
+    it('supports empty strings as default value', () => {
+      expect(doc.defaultEmptyString).toEqual('')
+    })
+
     it('should populate default values in beforeValidate hook', async () => {
       const { dependentOnFieldWithDefaultValue, fieldWithDefaultValue } = await payload.create({
         collection: 'text-fields',