EachKey: do not allocate array if there is no need

[xsandr]

Description: What this PR does
This PR reduces unnecessary array allocations in EachKey function.
In our application, we heavily use EachKey to traverse moderately big JSON objects and our benchmarks showed that this would reduce allocation up to 55%:

name                       old time/op    new time/op    delta
ParseRequest-8    84.7µs ± 2%    76.7µs ± 2%   -9.47%  (p=0.000 n=9+10)

name                       old alloc/op   new alloc/op   delta
ParseRequest-8    20.7kB ± 0%     9.3kB ± 0%  -55.15%  (p=0.000 n=10+10)

name                       old allocs/op  new allocs/op  delta
ParseRequest-8       370 ± 0%       192 ± 0%  -48.11%  (p=0.000 n=10+10)

Benchmark before change:

BenchmarkJsonParserLarge-8                        127772             46406 ns/op               0 B/op          0 allocs/op
BenchmarkJsonParserMedium
BenchmarkJsonParserMedium-8                       783925              7386 ns/op               0 B/op          0 allocs/op
BenchmarkJsonParserDeleteMedium
BenchmarkJsonParserDeleteMedium-8                 488871             13074 ns/op           12352 B/op          7 allocs/op
BenchmarkJsonParserEachKeyManualMedium
BenchmarkJsonParserEachKeyManualMedium-8          718426              8417 ns/op            4592 B/op         72 allocs/op
BenchmarkJsonParserEachKeyStructMedium
BenchmarkJsonParserEachKeyStructMedium-8          669814              9104 ns/op            5040 B/op         82 allocs/op
BenchmarkJsonParserObjectEachStructMedium
BenchmarkJsonParserObjectEachStructMedium-8       772330              7506 ns/op             448 B/op         10 allocs/op
BenchmarkJsonParserSmall
BenchmarkJsonParserSmall-8                       7662243               770 ns/op               0 B/op          0 allocs/op
BenchmarkJsonParserEachKeyManualSmall
BenchmarkJsonParserEachKeyManualSmall-8          6154617               998 ns/op             528 B/op          9 allocs/op
BenchmarkJsonParserEachKeyStructSmall
BenchmarkJsonParserEachKeyStructSmall-8          4874719              1209 ns/op             640 B/op         15 allocs/op
BenchmarkJsonParserObjectEachStructSmall
BenchmarkJsonParserObjectEachStructSmall-8       8326891               680 ns/op             112 B/op          6 allocs/op
BenchmarkJsonParserSetSmall
BenchmarkJsonParserSetSmall-8                    4711480              1222 ns/op             768 B/op          4 allocs/op
BenchmarkJsonParserDelSmall
BenchmarkJsonParserDelSmall-8                    2959894              2020 ns/op             976 B/op          8 allocs/op

Benchmark after change:

BenchmarkJsonParserLarge
BenchmarkJsonParserLarge-8                        130033             47934 ns/op               0 B/op          0 allocs/op
BenchmarkJsonParserMedium
BenchmarkJsonParserMedium-8                       773694              7482 ns/op               0 B/op          0 allocs/op
BenchmarkJsonParserDeleteMedium
BenchmarkJsonParserDeleteMedium-8                 484461             12878 ns/op           12352 B/op          7 allocs/op
BenchmarkJsonParserEachKeyManualMedium
BenchmarkJsonParserEachKeyManualMedium-8         1000000              5130 ns/op              48 B/op          1 allocs/op
BenchmarkJsonParserEachKeyStructMedium
BenchmarkJsonParserEachKeyStructMedium-8         1000000              5760 ns/op             496 B/op         11 allocs/op
BenchmarkJsonParserObjectEachStructMedium
BenchmarkJsonParserObjectEachStructMedium-8       712172              7493 ns/op             448 B/op         10 allocs/op
BenchmarkJsonParserSmall
BenchmarkJsonParserSmall-8                       7567230               764 ns/op               0 B/op          0 allocs/op
BenchmarkJsonParserEachKeyManualSmall
BenchmarkJsonParserEachKeyManualSmall-8          8969059               654 ns/op              16 B/op          1 allocs/op
BenchmarkJsonParserEachKeyStructSmall
BenchmarkJsonParserEachKeyStructSmall-8          6836643               836 ns/op             128 B/op          7 allocs/op
BenchmarkJsonParserObjectEachStructSmall
BenchmarkJsonParserObjectEachStructSmall-8       8643297               686 ns/op             112 B/op          6 allocs/op
BenchmarkJsonParserSetSmall
BenchmarkJsonParserSetSmall-8                    4850605              1240 ns/op             768 B/op          4 allocs/op
BenchmarkJsonParserDelSmall
BenchmarkJsonParserDelSmall-8                    2977360              1976 ns/op             976 B/op          8 allocs/op

[buger]

Nice one!