Fix UnmarshalJSON required check for nullable additionalProperties containers (#3762)

When a field is a nullable `map[string]interface{}` (object with additionalProperties),
the template was incorrectly emitting `.IsSet()` which only works on NullableXxx wrapper
types. Use `== nil` instead for additionalProperties container fields.

Co-authored-by: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: nogates

.generator/src/generator/templates/model_simple.j2

@@ -357,7 +357,8 @@ func (o *{{ name }}) UnmarshalJSON(bytes []byte) (err error) {
 	}
 	{%- for attr, spec in model.get("properties", {}).items() if attr in model.get("required", []) %}
 	{%- set propertyName = attr|attribute_name %}
-	{%- if not spec.nullable %}
+	{%- set isAdditionalPropertiesContainer = spec.type == "object" and spec.additionalProperties is defined and spec.additionalProperties is not false and not spec.get("properties") %}
+	{%- if not spec.nullable or isAdditionalPropertiesContainer %}
 	if all.{{ propertyName }} == nil {
 	{%- else %}
 	if !all.{{ propertyName }}.IsSet() {