do not allow users to set required parameters to NULL

Author: mattpollock

modules/openapi-generator/src/main/resources/r/api.mustache

@@ -192,7 +192,7 @@
       is_oauth <- FALSE
 
       {{#requiredParams}}
-      if (missing(`{{paramName}}`)) {
+      if (missing(`{{paramName}}`) || is.null(`{{paramName}}`)) {
       {{#useDefaultExceptionHandling}}
         stop("Missing required parameter `{{{paramName}}}`.")
       {{/useDefaultExceptionHandling}}