sdk/trace: empty OTEL_TRACES_SAMPLER* values are not treated as unset

[RedZapdos123]

Description:

Empty OTEL_TRACES_SAMPLER and OTEL_TRACES_SAMPLER_ARG values are not treated the same way as unset values in sdk/trace.

On current main, the SDK still falls back to the effective default sampler behavior, but it also emits configuration errors for empty values:

• OTEL_TRACES_SAMPLER="" logs unsupported sampler:
• OTEL_TRACES_SAMPLER_ARG="" with OTEL_TRACES_SAMPLER=traceidratio logs a parse error for the empty string

This appears to violate the environment variable spec requirement that empty values be interpreted the same way as unset values.

Steps to Reproduce:

1. Check out main at b34ee26cf3ea1c56e86f1e546d19c3aea5ded379.
2. From WSL in the repository root, run:

   repo=$(pwd)
   root=/tmp/otel-empty-sampler-repro
   rm -rf "$root"
   mkdir -p "$root"
   
   cat >"$root/go.mod" <<EOF
   module example.com/emptysamplerrepro
   
   go 1.25.0
   
   require (
       go.opentelemetry.io/otel v0.0.0
       go.opentelemetry.io/otel/sdk v0.0.0
   )
   
   replace go.opentelemetry.io/otel => $repo
   replace go.opentelemetry.io/otel/sdk => $repo/sdk
   replace go.opentelemetry.io/otel/trace => $repo/trace
   replace go.opentelemetry.io/otel/metric => $repo/metric
   EOF
   
   cat >"$root/main.go" <<'EOF'
   package main
   
   import (
       "fmt"
   
       "go.opentelemetry.io/otel"
       sdktrace "go.opentelemetry.io/otel/sdk/trace"
   )
   
   func main() {
       otel.SetErrorHandler(otel.ErrorHandlerFunc(func(err error) {
           fmt.Println("otel error:", err)
       }))
       _ = sdktrace.NewTracerProvider()
       fmt.Println("done")
   }
   EOF
   
   cd "$root"
   go mod tidy >/dev/null
   
   OTEL_TRACES_SAMPLER= go run .
   OTEL_TRACES_SAMPLER=traceidratio OTEL_TRACES_SAMPLER_ARG= go run .

Actual Behavior:

Both runs complete, but they log configuration errors even though the env var values are empty:

otel error: unsupported sampler: 
done
otel error: parsing sampler argument: strconv.ParseFloat: parsing "": invalid syntax
done

Expected Behavior:

Empty OTEL_TRACES_SAMPLER and OTEL_TRACES_SAMPLER_ARG values should be treated the same as if those variables were unset.

That means:

• OTEL_TRACES_SAMPLER="" should behave like the variable is absent: no config error, default sampler behavior.
• OTEL_TRACES_SAMPLER_ARG="" should behave like the variable is absent: no config error, sampler uses its unset/default argument behavior.

Notes:

Spec text at https://opentelemetry.io/docs/specs/otel/configuration/sdk-environment-variables/ says:

• empty env values must be interpreted the same as unset values
• OTEL_TRACES_SAMPLER_ARG invalid input must be ignored after logging, but empty input should first hit the empty-value rule

Current code path seems to come from sdk/trace/sampler_env.go:

• os.LookupEnv is used before trimming OTEL_TRACES_SAMPLER, so an empty value still enters sampler parsing
• os.LookupEnv also makes an empty OTEL_TRACES_SAMPLER_ARG count as present, so traceidratio tries to parse ""
• sdk/trace/provider.go then passes those returned errors to otel.Handle

[RedZapdos123]

Opened a PR to resolve the issue.