Address code review comments

Author: millicentachieng

src/Microsoft.OpenApi/Models/OpenApiDocument.cs

@@ -208,10 +208,13 @@ public void SerializeAsV2(IOpenApiWriter writer)
                     });
             }
             // parameters
-            IDictionary<string, OpenApiParameter> parameters =  Components?.Parameters;
+            var parameters = Components?.Parameters != null 
+                ? new Dictionary<string, OpenApiParameter>(Components.Parameters) 
+                : new Dictionary<string, OpenApiParameter>();
+
             if (Components?.RequestBodies != null)
             {
-                foreach (var requestBody in Components.RequestBodies)
+                foreach (var requestBody in Components.RequestBodies.Where(b => !parameters.ContainsKey(b.Key)))
                 {
                     parameters.Add(requestBody.Key, requestBody.Value.ConvertToBodyParameter());
                 }

src/Microsoft.OpenApi/Models/OpenApiOperation.cs

@@ -261,7 +261,11 @@ public void SerializeAsV2(IOpenApiWriter writer)
                         });
 
                     if (RequestBody.Reference.HostDocument != null)
-                        consumes = RequestBody.GetEffective(RequestBody.Reference.HostDocument)?.Content.Keys.Distinct().ToList();
+                    {
+                        var effectiveRequestBody = RequestBody.GetEffective(RequestBody.Reference.HostDocument);
+                        if (effectiveRequestBody != null)
+                            consumes = effectiveRequestBody.Content.Keys.Distinct().ToList();
+                    }
                 }
 
                 if (consumes.Any())

src/Microsoft.OpenApi/Models/OpenApiRequestBody.cs

@@ -169,6 +169,9 @@ internal OpenApiBodyParameter ConvertToBodyParameter()
 
         internal IEnumerable<OpenApiFormDataParameter> ConvertToFormDataParameters()
         {
+            if (Content == null || !Content.Any())
+                yield break;
+
             foreach (var property in Content.First().Value.Schema.Properties)
             {
                 var paramSchema = property.Value;
@@ -180,11 +183,11 @@ internal IEnumerable<OpenApiFormDataParameter> ConvertToFormDataParameters()
                     paramSchema.Format = null;
                 }
                 yield return new OpenApiFormDataParameter
-                { 
+                {
                     Description = property.Value.Description,
                     Name = property.Key,
                     Schema = property.Value,
-                    Required = Content.First().Value.Schema.Required.Contains(property.Key)
+                    Required = Content.First().Value.Schema.Required.Contains(property.Key) 
                 };
             }
         }