fix: content-length is 0 when responding with nil value

[abemedia]

No description provided.

[github-actions]

Benchmark Results

.

goos: linux
goarch: amd64
pkg: github.com/abemedia/go-don
cpu: Intel(R) Xeon(R) CPU E5-2673 v4 @ 2.30GHz
                         │   before    │               after                │
                         │   sec/op    │   sec/op     vs base               │
Handler/Request-2          427.4n ± 2%   442.5n ± 3%  +3.56% (p=0.017 n=10)
Handler/RequestPointer-2   436.1n ± 3%   462.2n ± 3%  +6.00% (p=0.000 n=10)
geomean                    431.7n        452.3n       +4.77%

                         │   before   │                after                │
                         │    B/op    │    B/op     vs base                 │
Handler/Request-2          24.00 ± 0%   24.00 ± 0%       ~ (p=1.000 n=10) ¹
Handler/RequestPointer-2   24.00 ± 0%   24.00 ± 0%       ~ (p=1.000 n=10) ¹
geomean                    24.00        24.00       +0.00%
¹ all samples are equal

                         │   before   │                after                │
                         │ allocs/op  │ allocs/op   vs base                 │
Handler/Request-2          1.000 ± 0%   1.000 ± 0%       ~ (p=1.000 n=10) ¹
Handler/RequestPointer-2   1.000 ± 0%   1.000 ± 0%       ~ (p=1.000 n=10) ¹
geomean                    1.000        1.000       +0.00%
¹ all samples are equal