Proposed text for AttributeValue clarification/cleanup

["Scott Cantor" <cantor.2@osu.edu>]

I discussed on the focus call clarifying/enabling handling of the three
"edge" cases for SAML Attribute values:

- no/zero values
- null values
- empty string values

The consensus was that all three should be more precisely defined. I propose
these changes to CD-1a:

Section 1.2.1, line 268, add clarification that the rule about strings
requiring one non-ws character be applicable "Unless otherwise noted in this
specification or particular profiles...."

Also in that section, suggest we clarify references to XML attributes with
the "XML" qualifier (just something good to do throughout the spec, and
we've been trying to do this as we find them, to avoid confusion).

Line 1164, clarify that by omitting <AttributeValue> we mean that the
attribute has "no values" (current text reads "no value", which might be
slightly ambiguous with empty or null values).

Insert at line 1191:
<<<
If a SAML attribute includes an empty value, such as the empty string, the
corresponding <AttributeValue> element MUST be empty (generally this is
serialized as <AttributeValue/>). This overrides the requirement in section
1.2.1 that string values in SAML content contain at least one non-whitespace
character.

If a SAML attribute includes a "null" value, the corresponding
<AttributeValue> element MUST be empty and MUST contain the reserved xsi:nil
XML attribute with a value of "true".
>>>

Line 1192, add nillable="true" to definition of <AttributeValue> element.

-- Scott