fix:fix NPE when rate-limiting with null value. (#669)

Signed-off-by: Haotian Zhang <[email protected]>

Author: SkyeBeFreeman

polaris-ratelimit/polaris-ratelimit-client/src/main/java/com/tencent/polaris/ratelimit/client/flow/QuotaFlow.java

@@ -277,6 +277,9 @@ private static String getLabelEntry(MatchArgument matchArgument, String labelVal
 
     private static String getLabelValue(MatchArgument matchArgument,
                                         Map<String, String> stringStringMap) {
+        if (CollectionUtils.isEmpty(stringStringMap)) {
+            return null;
+        }
         switch (matchArgument.getType()) {
             case CUSTOM:
             case HEADER: