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

[balajis-qb]

Description

Linked issue: Hacker0x01#6298

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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)

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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

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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

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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.