[core-rest-pipeline] Fix serialization of body of false (Azure#14218)

If the body contains just false, this check will confuse it with the body being non existent. This PR fixes this issue.

Author: deyaaeldeen

sdk/core/core-client/src/serializationPolicy.ts

@@ -127,12 +127,17 @@ export function serializeRequestBody(
       xmlName,
       xmlElementName,
       xmlNamespace,
-      xmlNamespacePrefix
+      xmlNamespacePrefix,
+      nullable
     } = bodyMapper;
     const typeName = bodyMapper.type.name;
 
     try {
-      if (request.body || required) {
+      if (
+        (request.body !== undefined && request.body !== null) ||
+        (nullable && request.body === null) ||
+        required
+      ) {
         const requestBodyParameterPathString: string = getPathStringFromParameter(
           operationSpec.requestBody
         );