fix(dynamite): always require content for a request body

Signed-off-by: Nikolas Rimikis <leptopoda@users.noreply.github.com>

Author: Leptopoda

packages/dynamite/dynamite/lib/src/builder/client.dart

@@ -232,7 +232,7 @@ Iterable<Method> buildTags(
       ({String mimeType, TypeResult result})? bodyParameter;
       final requestBody = operation.requestBody;
       if (requestBody != null) {
-        for (final content in requestBody.content!.entries) {
+        for (final content in requestBody.content.entries) {
           if (bodyParameter != null) {
             print('Can not work with multiple mime types right now. Using the first supported.');
             break;

packages/dynamite/dynamite/lib/src/models/openapi/request_body.dart

@@ -15,7 +15,7 @@ abstract class RequestBody implements Built<RequestBody, RequestBodyBuilder> {
   @BuiltValueField(compare: false)
   String? get description;
 
-  BuiltMap<String, MediaType>? get content;
+  BuiltMap<String, MediaType> get content;
 
   bool get required;