Fix NRT annotations for XmlProcessingInstruction

[krwq]

Fixes #71862

When we originally annotated XML we avoided any product changes which resulted in some annotations being incorrect.

@SteveDunn has found one of such inconsistencies when annotation S.S.Cryptography.Xml (#67198) and reported. This is fixing this inconsistency and matches behavior with spec and implementation.

Per https://www.w3.org/TR/2006/REC-xml11-20060816/#sec-pi:

• target cannot be null - current implementation will throw when it encounters null target: I'm changing the behavior to throw and making it explicitly non-nullable
• data - per spec this is optional although for current implementation there is no distinction between empty and null and current implementation return empty string when data is not provided: I'm changing the behavior that all getters for Data/Value (and whatever else it's called) are non-nullable and all setters are nullable - null will always translate to empty string since per spec it's equivalent and will make it most consistent with current implementation

cc: @SteveDunn

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Issue Details

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Fixes #71862

When we originally annotated XML we avoided any product changes which resulted in some annotations being incorrect.

@SteveDunn has found one of such inconsistencies when annotation S.S.Cryptography.Xml (#67198) and reported. This is fixing this inconsistency and matches behavior with spec and implementation.

Per https://www.w3.org/TR/2006/REC-xml11-20060816/#sec-pi:

• target cannot be null - current implementation will throw when it encounters null target: I'm changing the behavior to throw and making it explicitly non-nullable
• data - per spec this is optional although for current implementation there is no distinction between empty and null and current implementation return empty string when data is not provided: I'm changing the behavior that all getters for Data/Value (and whatever else it's called) are non-nullable and all setters are nullable - null will always translate to empty string since per spec it's equivalent and will make it most consistent with current implementation

cc: @SteveDunn

Author:	krwq
Assignees:	-
Labels:	area-System.Xml
Milestone:	-

[eerhardt]

LGTM to me. Just one small nit.