fix(core): correct useCustom return type to allow undefined data (fixes #7088)

[harikapadia999]

Description

Fixes #7088

The useCustom hook's result.data type was incorrectly typed as always defined, causing TypeScript errors when using optional chaining or checking for undefined values.

Changes

Type System

• Updated UseCustomReturnType to explicitly mark result.data as TData | undefined
• Added comprehensive JSDoc documentation explaining when data can be undefined
• Removed EMPTY_OBJECT fallback that masked the undefined state

Runtime Behavior

• Now correctly returns undefined for result.data during:
  • Loading state (query is pending)
  • Error state (query failed)
  • Disabled queries (enabled: false)
  • Before first execution

Testing

• Added 8 comprehensive test cases covering:
  • Undefined data during loading
  • Undefined data when query is disabled
  • Undefined data on error
  • Empty array vs undefined distinction
  • Null data handling
  • Optional chaining safety
  • State transition from undefined to defined

Impact

✅ Enables proper null-safety checks - Users can now safely use optional chaining
✅ Aligns types with runtime - TypeScript types now match actual behavior
✅ Backward compatible - Existing code continues to work
✅ Better DX - No more type errors for correct code

Example

Before (Type Error):

const { result } = useCustom({ url: "/api/data", method: "get" });
const items = result.data?.items; // ❌ Type error: data is never undefined

After (Works Correctly):

const { result } = useCustom({ url: "/api/data", method: "get" });
const items = result.data?.items; // ✅ Works! data can be undefined

[changeset-bot]

🦋 Changeset detected

Latest commit: d65825e

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

Name	Type
@refinedev/core	Patch

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