fix(encode): reject a nil MarshalTOML result

Assisted-by: GLM 5.3

encode.go

@@ -300,6 +300,12 @@ func buildMapDoc(v reflect.Value, doc *tomlDoc, ctx string) error {
 
 // --- reflection walk: field dispatch -------------------------------------
 
+// errNilMarshalTOML reports a Marshaler whose method returned a nil value
+// with no error. nil has no TOML representation, so dropping the field
+// silently or panicking on the invalid reflect.Value would both hide the
+// contract violation.
+var errNilMarshalTOML = errors.New("MarshalTOML returned a nil value")
+
 func addField(doc *tomlDoc, name string, v reflect.Value, ctx string) error {
 	if v.CanInterface() {
 		if m, ok := v.Interface().(Marshaler); ok {
@@ -307,6 +313,9 @@ func addField(doc *tomlDoc, name string, v reflect.Value, ctx string) error {
 			if err != nil {
 				return &EncodeError{Path: joinKey(ctx, name), Err: err}
 			}
+			if mv == nil {
+				return &EncodeError{Path: joinKey(ctx, name), Err: errNilMarshalTOML}
+			}
 			v = reflect.ValueOf(mv)
 		}
 	}
@@ -448,6 +457,9 @@ func addArrayValue(doc *tomlDoc, name string, v reflect.Value, ctx string) error
 				if err != nil {
 					return &EncodeError{Path: fmt.Sprintf("%s[%d]", joinKey(ctx, name), i), Err: err}
 				}
+				if mv == nil {
+					return &EncodeError{Path: fmt.Sprintf("%s[%d]", joinKey(ctx, name), i), Err: errNilMarshalTOML}
+				}
 				ev = reflect.ValueOf(mv)
 				ev = followPtr(ev)
 			}
@@ -474,7 +486,14 @@ func normaliseValue(v reflect.Value) (any, error) {
 	}
 	if v.CanInterface() {
 		if m, ok := v.Interface().(Marshaler); ok {
-			return m.MarshalTOML()
+			mv, err := m.MarshalTOML()
+			if err != nil {
+				return nil, err
+			}
+			if mv == nil {
+				return nil, errNilMarshalTOML
+			}
+			return mv, nil
 		}
 	}
 	// The datetime structs are TOML scalars; the emitter renders each of them.