[dart-dio] Fix json_serializable serialize template to support hasFormParams

The `json_serializable` dart-dio serialize template only handled
`{{#bodyParam}}`, which meant operations using
`application/x-www-form-urlencoded` or `multipart/form-data` produced an
empty `try {}` block and a `_bodyData` that was never assigned. The
request body was silently dropped.

This mirrors the handling already present in the `built_value` template
for the same generator, but without `built_value`-specific
`encodeFormParameter` / `_serializers` calls since `json_serializable`
passes values directly (consistent with its `query_param.mustache`).

- `application/x-www-form-urlencoded` -> `Map<String, dynamic>`
- `multipart/form-data` -> `FormData.fromMap(<String, dynamic>{...})`
- Optional / non-required + non-nullable params are conditionally
  included with `if (paramName != null)`.
- The existing `bodyParam` branch is preserved, just properly indented
  (to match `built_value`).

Author: Bazni

modules/openapi-generator/src/main/resources/dart/libraries/dio/serialization/json_serializable/api/serialize.mustache

@@ -1 +1,19 @@
-{{#bodyParam}}_bodyData=jsonEncode({{{paramName}}});{{/bodyParam}}
+      {{#hasFormParams}}
+      {{#isMultipart}}
+      _bodyData = FormData.fromMap(<String, dynamic>{
+        {{#formParams}}
+        {{^required}}{{^isNullable}}if ({{{paramName}}} != null) {{/isNullable}}{{/required}}r'{{{baseName}}}': {{#isFile}}{{{paramName}}}{{#isArray}}.toList(){{/isArray}}{{/isFile}}{{^isFile}}{{{paramName}}}{{/isFile}},
+        {{/formParams}}
+      });
+      {{/isMultipart}}
+      {{^isMultipart}}
+      _bodyData = <String, dynamic>{
+        {{#formParams}}
+        {{^required}}{{^isNullable}}if ({{{paramName}}} != null) {{/isNullable}}{{/required}}r'{{{baseName}}}': {{{paramName}}},
+        {{/formParams}}
+      };
+      {{/isMultipart}}
+      {{/hasFormParams}}
+      {{#bodyParam}}
+      _bodyData = jsonEncode({{{paramName}}});
+      {{/bodyParam}}