fix(api): Flatten ScoreComponent to empty string if score value is null

[Konicai]

The ScoreComponent value is nullable. When the value is null (which it always should be for 1.16+) two problems occur:

• FlattenerListener#component(@NotNull String text) implemented by the LegacyTextSerializer rightfully throws an NPE.
• PlainTextSerializer serializes the ScoreComponent as a string literal null, unlike the vanilla client.

This attempts to resemble client behaviour more closely:

• <1.16: Display the score value, otherwise empty text.
• 1.16+: Always display empty text. This PR does not match that, as it will still display the value regardless of game version.

Closes #953