Windows accessibility crash/assert in Gallery example

MSVC 2022, Win11, debug

Reproduce:

1. Run Widgets example
2. Start narrator
3. Click in password widget

Expected behavior:

• Cursor is placed in QLineEdit

Actual behavior:

• Assert in QWindowsUiaTextRangeProvider::ExpandToEnclosingUnit

Comment:

QWindowsUiaTextRangeProvider::ExpandToEnclosingUnit does not take into account that QAccessibleTextInterface::text() may return null QString even if QAccessibleTextInterface::characterCount returns non-zero. text() can return null QString if echo-mode is set to Password.