[vishr]

Summary

Fixes #3169.

BindHeaders indexes nil or zero-length header value slices when binding scalar map values or tagged struct fields. Treat entries with no values as absent for those destinations, while preserving slice-map values and custom multi-value unmarshalling.

- nil / []string{} -> values[0] -> panic
+ nil / []string{} -> skip scalar binding
  []string{""}     -> bind empty string

Related: #3170, #3171, #3175, and #3178. This version preserves map[string][]string entries and passes empty values to custom UnmarshalParams implementations. It also avoids allocating ordinary pointer fields for absent values.

Evidence

• Before: TestBindHeadersEmptyValues panics on nil and empty slices for tagged structs, map[string]string, and map[string]any.
  After: existing scalar values remain unchanged, one empty string still binds, slice-map values survive, ordinary pointer fields stay nil, and custom multi-value unmarshalling is still called.
• Independent branch: GOTOOLCHAIN=go1.27.4 go test -race ./..., go vet ./..., golint -set_exit_status ./..., and current staticcheck pass.

Merge Danger

Door: two-way

Blast Radius: binding

The shared binder skips scalar entries with no values. The reported header state requires in-process construction or mutation; this PR does not claim a remotely exploitable denial of service.