RFR: 8361717: Refactor Collections.emptyList() in Locale related classes

[Johannes Döbler]

On Wed, 9 Jul 2025 18:39:40 GMT, Naoto Sato <naoto at openjdk.org> wrote:

>  modernizing the code by using List.of() is still a desirable improvement

except that `Collections.emptyList()` and `List.of()` unfortunately have different tolerance to calls `List.indexOf(null)` and `List.contains(null)`.

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PR Comment: https://git.openjdk.org/jdk/pull/26225#issuecomment-3053718956