encoding/xml: require whitespace before processing instruction value

[DemiMarie]

The value of a processing instruction must either be empty or have whitespace before it.

Fixes #68386

[gopherbot]

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[gopherbot]

Message from Gopher Robot:

Patch Set 1:

(1 comment)

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Please don’t reply on this GitHub thread. Visit golang.org/cl/609379.
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[gopherbot]

Message from Demi Obenour:

Patch Set 1:

(1 comment)

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Please don’t reply on this GitHub thread. Visit golang.org/cl/609379.
After addressing review feedback, remember to publish your drafts!