GRAILS-6618 - only clear the params if there are any

Author: jeffscottbrown

src/java/org/codehaus/groovy/grails/web/metaclass/ForwardMethod.groovy

@@ -46,7 +46,11 @@ class ForwardMethod {
     }
 
     String forward(HttpServletRequest request, HttpServletResponse response, Map params) {
-		request.parameterMap?.clear()
+		def paramMap = request.parameterMap
+		if(paramMap) {
+			paramMap.clear()
+		}
+
         def urlInfo = new ForwardUrlMappingInfo()
 
         GrailsWebRequest webRequest = GrailsWebRequest.lookup(request)