10788: ApiClient for resttemplate should not produce warning (#10789)

Motivation:

Remove useless warnings for webclient and resttemplate

samples/client/petstore/java/webclient-nulable-arrays/src/main/java/org/openapitools/client/ApiClient.java

@@ -424,6 +424,7 @@ public class ApiClient extends JavaTimeFormatter {
         }
 
         if (value instanceof Map) {
+            @SuppressWarnings("unchecked")
             final Map<String, Object> valuesMap = (Map<String, Object>) value;
             for (final Entry<String, Object> entry : valuesMap.entrySet()) {
                 params.add(entry.getKey(), parameterToString(entry.getValue()));