[security-services] Final text for DS-4-13 (empty strings/URIs)

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

Per the AI, here is text to be added as new section 1.2.1 (in front of
the current 1.2.1 on Time Values):

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1.2.1 String and URI Values

All SAML string and URI values have the types string and anyURI
respectively, which are built in to the W3C XML Schema Datatypes
specification. All strings in SAML messages MUST consist of at least one
non-whitespace character (whitespace is defined in [XML 1.0 Sec. 2.3]).
Empty and whitespace-only values are disallowed. Also, unless otherwise
indicated in this specification, all URI values MUST consist of at least
one non-whitespace character.

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Also, to clarify the use of an empty URI as a Resource, add the
following text to lines 943 and 1311:

"It is permitted for this attribute to have the value of the empty URI
reference (""), and the meaning is defined to be "the start of the
current document", as specified by RFC 2396 section 4.2."

-- Scott