Limit types that get converted to null for empty/null input in NullableConverter

[oroztocil]

Limits the nullable types for which empty string or null input gets converted to null in NullableConverter. After the change this only happens for types that either have TypeCode != TypeCode.Object or belong to a set of special supported types (currently DateOnly, TimeOnly, DateTimeOffset).

Other types get processed by the respective non-nullable converter as was the case before #52499. In particular, types implementing IParsable (with the exception of the types mentioned above) get converted using their TryParse method (via ParsableConverter) which means they can provide custom implementation for handling empty/null inputs.

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Comments suppressed due to low confidence (1)

src/Components/Endpoints/test/FormMapping/Converters/NullableConverterTests.cs:210

• Consider adding an assertion that verifies result is null when conversion fails, to make the expected behavior more explicit.

var returnValue = nullableConverter.TryConvertValue(ref reader, "bad value", out var result);