Handle Nil response body from SigV4

[lewinkedrs]

Previously in #746 we added the capability for SigV4 support. This worked but did not handle the case where the response for a certain query was Nil. This caused a panic. In this change we handle the Nil case to prevent panic.

[lewinkedrs]

Just to provide some context, here is the panic we got when would get a nil response body. I am also submitting this fix to prometheus/sigv4 to fix this in other places

goroutine 95 [running]:
bytes.(*Buffer).ReadFrom(0x14000a1e780, {0x0, 0x0})
        /Users/lewinke/go/pkg/mod/golang.org/toolchain@v0.0.1-go1.24.6.darwin-arm64/src/bytes/buffer.go:211 +0x6c
io.copyBuffer({0x103a17a40, 0x14000a1e780}, {0x0, 0x0}, {0x0, 0x0, 0x0})
        /Users/lewinke/go/pkg/mod/golang.org/toolchain@v0.0.1-go1.24.6.darwin-arm64/src/io/io.go:415 +0x14c
io.Copy(...)
        /Users/lewinke/go/pkg/mod/golang.org/toolchain@v0.0.1-go1.24.6.darwin-arm64/src/io/io.go:388
github.com/prometheus/common/sigv4.(*sigV4RoundTripper).RoundTrip(0x140006dcc80, 0x14000888780)
        /Users/lewinke/promxy/vendor/github.com/prometheus/common/sigv4/sigv4.go:105 +0xcc
github.com/jacksontj/promxy/pkg/servergroup.(*ServerGroup).RoundTrip(0x14000484690, 0x14000888780)
        /Users/lewinke/promxy/pkg/servergroup/servergroup.go:114 +0x324
net/http.send(0x14000888780, {0x103a19bc0, 0x14000484690}, {0x10070e878?, 0x8?, 0x0?})
        /Users/lewinke/go/pkg/mod/golang.org/toolchain@v0.0.1-go1.24.6.darwin-arm64/src/net/http/client.go:259 +0x498
net/http.(*Client).send(0x1400092e048, 0x14000888780, {0x1069d6fc0?, 0x100495ec0?, 0x0?})
        /Users/lewinke/go/pkg/mod/golang.org/toolchain@v0.0.1-go1.24.6.darwin-arm64/src/net/http/client.go:180 +0x94
net/http.(*Client).do(0x1400092e048, 0x14000888780)
        /Users/lewinke/go/pkg/mod/golang.org/toolchain@v0.0.1-go1.24.6.darwin-arm64/src/net/http/client.go:728 +0x764
net/http.(*Client).Do(...)
        /Users/lewinke/go/pkg/mod/golang.org/toolchain@v0.0.1-go1.24.6.darwin-arm64/src/net/http/client.go:587
github.com/prometheus/client_golang/api.(*httpClient).Do(0x1400092e040, {0x103a4f560, 0x140006e6460}, 0x14000888640?)
        /Users/lewinke/promxy/vendor/github.com/prometheus/client_golang/api/client.go:125 +0xf4
github.com/prometheus/client_golang/api/prometheus/v1.(*apiClientImpl).Do(0x103a4f4f0?, {0x103a4f560?, 0x140006e6460?}, 0x4?)
        /Users/lewinke/promxy/vendor/github.com/prometheus/client_golang/api/prometheus/v1/api.go:1365 +0x34
github.com/prometheus/client_golang/api/prometheus/v1.(*httpAPI).LabelValues(0x14000620710, {0x103a4f560, 0x140006e6460}, {0x14000a32012, 0x8}, {0x105767620, 0x0, 0x102779318?}, {0x14000923a38?, 0x14000a1e510?, ...}, ...)
        /Users/lewinke/promxy/vendor/github.com/prometheus/client_golang/api/prometheus/v1/api.go:1061 +0x430
github.com/jacksontj/promxy/pkg/promclient.(*PromAPIV1).LabelValues(0x14000620720, {0x103a4f560, 0x140006e6460}, {0x14000a32012, 0x8}, {0x105767620, 0x0, 0x0}, {0x14000923af8?, 0x5?, ...}, ...)
        /Users/lewinke/promxy/pkg/promclient/api.go:56 +0x124
github.com/jacksontj/promxy/pkg/promclient.(*AddLabelClient).LabelValues(0x140008ff008, {0x103a4f560, 0x140006e6460}, {0x14000a32012, 0x8}, {0x0?, 0x0?, 0x0?}, {0x0?, 0xffdf3b8149409a0a?, ...}, ...)
        /Users/lewinke/promxy/pkg/promclient/label.go:135 +0xbc

[lewinkedrs]

Tests Passing:

❯ go test ./...
?       github.com/jacksontj/promxy     [no test files]
?       github.com/jacksontj/promxy/cmd/promxy  [no test files]
?       github.com/jacksontj/promxy/cmd/remote_write_exporter   [no test files]
ok      github.com/jacksontj/promxy/pkg/alertbackfill   (cached)
ok      github.com/jacksontj/promxy/pkg/config  (cached)
ok      github.com/jacksontj/promxy/pkg/logging (cached)
?       github.com/jacksontj/promxy/pkg/middleware      [no test files]
ok      github.com/jacksontj/promxy/pkg/promclient      (cached)
ok      github.com/jacksontj/promxy/pkg/promhttputil    (cached)
?       github.com/jacksontj/promxy/pkg/proxyquerier    [no test files]
ok      github.com/jacksontj/promxy/pkg/proxystorage    (cached)
ok      github.com/jacksontj/promxy/pkg/remote  (cached)
ok      github.com/jacksontj/promxy/pkg/server  (cached)
ok      github.com/jacksontj/promxy/pkg/servergroup     (cached)
ok      github.com/jacksontj/promxy/test        (cached)

[lewinkedrs]

@pedroapero would you be able to take a look at this? Small change to handle an edge case from the SigV4 work.