Allow nil predicates, matching all nodes

[bobg]

This PR adds the ability to specify nil as the value for any node predicate. It's equivalent to specifying

func (*html.Node) bool { return true }

[github-actions]

Modver result

This report was generated by Modver,
a Go package and command that helps you obey semantic versioning rules in your Go module.

This PR does not require a change in your module’s version number.
(You might still consider bumping the patchlevel anyway.)

[coveralls]

Pull Request Test Coverage Report for Build 12771077926

Details

• 4 of 4 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 75.385%

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Totals
Change from base Build 12741303877:	0.0%
Covered Lines:	196
Relevant Lines:	260

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💛 - Coveralls