fix: omitempty must not drop structs with non-zero unexported fields

[sonnemusk]

Problem

isEmptyValue decided whether a struct tagged omitempty was empty by scanning only exported fields (structs.Fields + OmitEmpty). A struct with zero-valued exported fields but non-zero unexported fields was incorrectly treated as empty and omitted, even though it is not the zero value of its type.

Minimal reproduction (#378)

type Val struct {
	content string
}

func (v Val) EncodeMsgpack(enc *msgpack.Encoder) error {
	return enc.EncodeString(v.content)
}

func (v *Val) DecodeMsgpack(dec *msgpack.Decoder) error {
	c, err := dec.DecodeString()
	if err != nil {
		return err
	}
	v.content = c
	return nil
}

type Foo struct {
	Val Val `msgpack:"val,omitempty"`
}

// Before: Val was omitted; round-trip lost "str".
foo := Foo{Val: Val{content: "str"}}

Fix

For reflect.Struct, use reflect.Value.IsZero, which checks all fields (exported and unexported). A struct is omitted only when it equals the zero value of its type.

Zero structs continue to be omitted; non-zero unexported state no longer causes data loss for exported fields or custom encoders.

Tests

• Regression for mixed exported/unexported fields
• Structs with only unexported fields
• CustomEncoder/CustomDecoder playground case from omitempty unexpectedly omits a member when the member is a struct with a non-zero unexported member #378

All existing tests pass, including ExampleMarshal_ignore_simple_zero_structs_when_tagged_with_omitempty.

Fixes #378

Note on #390

#390 discusses broader omitempty vs encoding/json semantics (interface/any emptiness, future omitzero). This PR only fixes the unexported-field empty check from #378 and does not change interface/any omitempty behavior.