Revert HeightParser behavior to NOT return an error if DAU has no value

[jonsagara]

The previous version of the parser returned null when the DAU field had no value, or its length was less than 3 characters:

if (string.IsNullOrEmpty(input) || input.Length < 3)
{
    return null;
}

The new version treats this as unparsed due to erroneous omission:

if (string.IsNullOrEmpty(rawValue) || rawValue.Length < 3)
{
    return FieldHelpers.UnparsedField<Height?>(elementId: elementId, rawValue: rawValue, $"Unable to parse Height from field '{SubfileElementIds.Height}': the field has no value, or has less than 3 characters: '{rawValue}'");
}

This causes problems for Michigan, which evidently allows the DAU element to be present without a value.

The AAMVA spec says Height (DAU) is a mandatory field, with this paragraph about missing data:

Mandatory data elements for which no data exists for a given cardholder are to be encoded with the word "NONE".
In the event data is not available for a mandatory data element, "unavl" is to be encoded.

MI apparently decided to rage against the machine and do neither of these things, and instead just leave the value blank. The original MI.txt test file has a blank DAU field.

To support this, split the "no value or length < 3" case into two separate clauses:

• When value is null or white space, treat it as successfully parsed, and set the value to null
• When value is less than 3 characters in length, treat it as an error because the field has a value, but we can't parse it