Address code review comments

Author: millicentachieng

src/Microsoft.OpenApi/Models/OpenApiDocument.cs

@@ -208,12 +208,13 @@ public void SerializeAsV2(IOpenApiWriter writer)
                     });
             }
             // parameters
-            IDictionary<string, OpenApiParameter> parameters =  Components?.Parameters;
+            var parameters = new Dictionary<string, OpenApiParameter>(Components?.Parameters);
             if (Components?.RequestBodies != null)
             {
                 foreach (var requestBody in Components.RequestBodies)
                 {
-                    parameters.Add(requestBody.Key, requestBody.Value.ConvertToBodyParameter());
+                    if (!parameters.ContainsKey(requestBody.Key))
+                        parameters.Add(requestBody.Key, requestBody.Value.ConvertToBodyParameter());
                 }
             }
             writer.WriteOptionalMap(

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

@@ -169,7 +169,10 @@ internal OpenApiBodyParameter ConvertToBodyParameter()
 
         internal IEnumerable<OpenApiFormDataParameter> ConvertToFormDataParameters()
         {
-            foreach (var property in Content.First().Value.Schema.Properties)
+            var properties = Content != null && Content.Any() 
+                ? Content.First().Value?.Schema?.Properties :
+                new Dictionary<string, OpenApiSchema>();
+            foreach (var property in properties)
             {
                 var paramSchema = property.Value;
                 if ("string".Equals(paramSchema.Type, StringComparison.OrdinalIgnoreCase)