Javadoc: TypeConverters convert for "Level" incorrectly documents behaviour for invalid value

[JWT007]

Log4j 2.24

Minor Javadoc correction.

In TypeConverters, the conversion for type "Level" says it returns 'null' for invalid levels:

/**
 * Converts a {@link String} into a Log4j {@link Level}. Returns {@code null} for invalid level names.
 */
@Plugin(name = "Level", category = CATEGORY)
public static class LevelConverter implements TypeConverter<Level> {
    @Override
    public Level convert(final String s) {
        return Level.valueOf(s);
    }
} 

But Level.valueOf(s) throws an IllegalArgumentException for unknown levels.

[JWT007]

Hi @ppkarwasz - another PR - this one should be an easy one :)