UnitParser: fix the nullability of the Parse / TryParse overloads

[lipchev]

Can we add this to #1200

• public Enum Parse(string? unitAbbreviation, Type unitType, IFormatProvider? formatProvider = null): unitAbbreviation should be non-nullable..
• public bool TryParse<TUnitType>(string unitAbbreviation, out TUnitType unit) where TUnitType : struct, Enum : unitAbbreviation should be nullable
• public bool TryParse(string unitAbbreviation, Type unitType, [NotNullWhen(true)] out Enum? unit): unitAbbreviation should be nullable

[lipchev]

Oh I just realized this also the case for the static method on the quantities (e.g. Mass.TryParseUnit(..)):

• public static bool TryParseUnit(string str, out MassUnit unit)
• public static bool TryParseUnit(string str, IFormatProvider? provider, out MassUnit unit)

[angularsen]

Added it