Fix #6298: 🐛 Allow Enter key selection when selected is null

[balajis-qb]

Description

Linked issue: #6298

---

Problem

In YearPicker (showYearPicker), pressing Enter does not select a year when selected is null or undefined.

This is caused by an early return in onYearKeyDown (src/year.tsx):

if (this.props.selected == null) {
  break;
}

This prevents keyboard-based selection entirely when no initial value is set.

The behavior is inconsistent with:

• Mouse selection (works correctly)
• Default DatePicker and MonthPicker (keyboard works even with null selection)

---

Changes

• Removed the selected == null guard from Enter key handling
• Enabled selection based on the currently focused/preselected year
• Updated test cases to reflect expected behavior
• Removed/adjusted test enforcing no selection when selected is null

---

Behavior Changes

• ✅ Enter key now selects a year even when selected is initially null
• ✅ Keyboard and mouse interactions are now consistent
• ✅ Behavior aligned with other picker modes
• ⚠️ Existing tests expecting no selection on null selected are updated

---

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.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.31%. Comparing base (548a1f3) to head (8008f4e).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6300      +/-   ##
==========================================
+ Coverage   99.29%   99.31%   +0.02%     
==========================================
  Files          30       30              
  Lines        3822     3820       -2     
  Branches     1648     1664      +16     
==========================================
- Hits         3795     3794       -1     
+ Misses         26       25       -1     
  Partials        1        1              

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