logfmt: bare keys get "true" as value, not "nil"

[seveas]

In #4359, @nokute78 notes that bare keys in logfmt lines get the wrong
value after parsing: they should get a true value, not nil.

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[seveas]

Test logline:

foo=bar baz=quux plonk1 plonk2 last_plonk

Old output

[0] tail.0: [1664193795.762467000, {"foo"=>"bar", "baz"=>"quux", "plonk1"=>nil, "plonk2"=>nil}]

New output

[0] tail.0: [1664194341.064513000, {"foo"=>"bar", "baz"=>"quux", "plonk1"=>true, "plonk2"=>true}]

(last_plonk is still missing from the output, #4359 will fix that)

[nokute78]

LGTM.

Note: I modified PR title to pass CI.

Note: I referred to following document.
https://pkg.go.dev/github.com/kr/logfmt#pkg-overview

Example logfmt message:
foo=bar a=14 baz="hello kitty" cool%story=bro f %^asdf

Example result in JSON:
{ "foo": "bar", "a": 14, "baz": "hello kitty", "cool%story": "bro", "f": true, "%^asdf": true }

[edsiper]

thansk!