feat: Handle a nullable map union w/ a plain map

Similar to how nullable slices are handles, this adds support for using
plain `map[string]any` for nullable unions of maps instead of using an
explicit pointer to a map.

README.md

@@ -115,7 +115,7 @@ When a non-`nil` union value is encountered, a single key is en/decoded. The key
 type name, or scheam full name in the case of a named schema (enum, fixed or record).
 * ***T:** This is allowed in a "nullable" union. A nullable union is defined as a two schema union, 
 with one of the types being `null` (ie. `["null", "string"]` or `["string", "null"]`), in this case 
-a `*T` is allowed, with `T` matching the conversion table above. In the case of a slice, the slice can be used
+a `*T` is allowed, with `T` matching the conversion table above. In the case of a slice or a map, the slice or the map can be used
 directly.
 * **any:** An `interface` can be provided and the type or name resolved. Primitive types
 are pre-registered, but named types, maps and slices will need to be registered with the `Register` function. 

codec_union.go

@@ -13,10 +13,18 @@ import (
 func createDecoderOfUnion(d *decoderContext, schema *UnionSchema, typ reflect2.Type) ValDecoder {
 	switch typ.Kind() {
 	case reflect.Map:
-		if typ.(reflect2.MapType).Key().Kind() != reflect.String ||
-			typ.(reflect2.MapType).Elem().Kind() != reflect.Interface {
+		if typ.(reflect2.MapType).Key().Kind() != reflect.String {
 			break
 		}
+
+		if typ.(reflect2.MapType).Elem().Kind() != reflect.Interface {
+			if !schema.Nullable() {
+				break
+			}
+
+			return decoderOfNullableUnion(d, schema, typ)
+		}
+
 		return decoderOfMapUnion(d, schema, typ)
 	case reflect.Slice:
 		if !schema.Nullable() {
@@ -44,10 +52,18 @@ func createDecoderOfUnion(d *decoderContext, schema *UnionSchema, typ reflect2.T
 func createEncoderOfUnion(e *encoderContext, schema *UnionSchema, typ reflect2.Type) ValEncoder {
 	switch typ.Kind() {
 	case reflect.Map:
-		if typ.(reflect2.MapType).Key().Kind() != reflect.String ||
-			typ.(reflect2.MapType).Elem().Kind() != reflect.Interface {
+		if typ.(reflect2.MapType).Key().Kind() != reflect.String {
 			break
 		}
+
+		if typ.(reflect2.MapType).Elem().Kind() != reflect.Interface {
+			if !schema.Nullable() {
+				break
+			}
+
+			return encoderOfNullableUnion(e, schema, typ)
+		}
+
 		return encoderOfMapUnion(e, schema, typ)
 	case reflect.Slice:
 		if !schema.Nullable() {
@@ -179,6 +195,8 @@ func decoderOfNullableUnion(d *decoderContext, schema Schema, typ reflect2.Type)
 		isPtr = true
 	case *reflect2.UnsafeSliceType:
 		baseTyp = v
+	case *reflect2.UnsafeMapType:
+		baseTyp = v
 	}
 	decoder := decoderOfType(d, union.Types()[typeIdx], baseTyp)
 
@@ -249,6 +267,8 @@ func encoderOfNullableUnion(e *encoderContext, schema Schema, typ reflect2.Type)
 		isPtr = true
 	case *reflect2.UnsafeSliceType:
 		baseTyp = v
+	case *reflect2.UnsafeMapType:
+		baseTyp = v
 	}
 	encoder := encoderOfType(e, union.Types()[typeIdx], baseTyp)