fix: embedded omitempty field dropped / nil-pointer panic in nested embedded structs (#576, #581, #554) - #587

Fixes #576. Also fixes the panic reported in #581 and #554.

When an embedded struct's first field is tagged omitempty and is empty, the whole containing struct serialized to {}, silently dropping every following field (#576). The same broken wiring also caused a nil-pointer panic for two closely related shapes (#581, #554).

The cause is in encoder.StructCode.ToAnonymousOpcode: prevField was set to firstField (the anonymous head opcode) rather than the last field of the embedded chain, so the omitempty field's NextField never linked to the next real field. addStructEndCode then wired that head's empty-skip jump straight to StructEnd. The non-anonymous ToOpcode path already does this correctly via lastFieldCode; this mirrors it.

// #576: dropped fields
type ErrOmit struct {
    Error string `json:"error,omitempty"`
}
type MyResult struct {
    ErrOmit
    Data string `json:"data"`
}
type Result struct{ MyResult }
// before: {}        encoding/json: {"data":""}

// #581: panic
type Reason struct {
    Code string `json:"reasonCode,omitempty"`
}
type Cov struct {
    Type string `json:"type"`
}
type Inner struct {
    Cov
    R *Reason `json:"reason,omitempty"`
}
type Outer struct{ Inner }
// before: panic (nil pointer)   encoding/json: {"type":""}

// #554: panic
type Son struct {
    B string `json:"b"`
    C string `json:"c"`
}
type Father struct {
    Son
    A []string `json:"a,omitempty"`
}
type Grandpa struct{ Father }
// before: panic (nil pointer)   encoding/json: {"b":"","c":""}

Added TestIssue576 (empty / data-only / both-fields), TestIssue581 (nil and set pointer), and TestIssue554 (double-nested embedded with trailing omitempty empty slice). Each was confirmed to fail before the change ({"data":""} -> {} for #576, panic for #581 and #554) and pass after. Full go test ./... is green, gofmt clean.

For anyone triaging the embedded-omitempty cluster: I dump-tested the neighbours against this branch. #512 is a distinct shape (a plain field followed by an embedded struct whose own field is omitempty, double-nested) that goes through a separate opcode path and still panics; not addressed here. #486 (nil embedded self-pointer emitting {"id":99,null}) and #503 (omitempty pointer child with a zero first byte) are also separate paths that survive this fix and remain open. Happy to follow those up in separate PRs.