fix: allow keyboard year selection without selected date

[wanxiankai]

Description

Fixes #6298

Problem

When showYearPicker is enabled without an initial selected date, the year cells could be clicked with the mouse but pressing Enter on a focused year did nothing. The keyboard handler returned early whenever selected was nullish, so keyboard-only users could not select a year.

Changes

• Route Enter through the same year-selection handler used by mouse clicks.
• Keep the existing pre-selection update for controlled pickers that already have a selected date.
• Add regression coverage for selecting a year with Enter when no date is selected, including the DatePicker integration path.

Verification

• yarn test src/test/year_picker_test.test.tsx --runInBand --watchman=false
• yarn type-check
• yarn eslint src/year.tsx src/test/year_picker_test.test.tsx
• yarn prettier --check src/year.tsx src/test/year_picker_test.test.tsx

Contribution checklist

• I have followed the contributing guidelines.
• I have added sufficient test coverage for my changes.
• I have formatted my code with Prettier and checked for linting issues with ESLint for code readability.