Fix Phonemizer ALT handling - #2259

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Nullability of reference types in type of parameter 'value' doesn't match implemented member 'int IList.Add(object? value)' (possibly because of nullability attributes).

Cannot convert null literal to non-nullable reference type.

Cannot convert null literal to non-nullable reference type.

Cannot convert null literal to non-nullable reference type.

'JapaneseMonophoneG2p.LoadPack(byte[], Func<string, string>, Func<string, string>)' hides inherited member 'G2pPack.LoadPack(byte[], Func<string, string>, Func<string, string>)'. Use the new keyword if hiding was intended.

Cannot convert null literal to non-nullable reference type.

Cannot convert null literal to non-nullable reference type.

Cannot convert null literal to non-nullable reference type.

Cannot convert null literal to non-nullable reference type.