Fix AccountLimitsConfig error prone error

The following error was fixed:
  error: [ImpossibleNullComparison] This value cannot be null, and comparing it to null may be misleading.
    return Optional.ofNullable(rateLimits.row(type));
                              ^

Change-Id: Iffc891f138b70bbfc38c004f1081476e4c7569ec

src/main/java/com/googlesource/gerrit/plugins/quota/AccountLimitsConfig.java

diff --git a/src/main/java/com/googlesource/gerrit/plugins/quota/AccountLimitsConfig.java b/src/main/java/com/googlesource/gerrit/plugins/quota/AccountLimitsConfig.java
index 9c94fae..504dd2f 100644
--- a/src/main/java/com/googlesource/gerrit/plugins/quota/AccountLimitsConfig.java
+++ b/src/main/java/com/googlesource/gerrit/plugins/quota/AccountLimitsConfig.java
@@ -188,9 +188,6 @@
    * @return map of rate limits per group name
    */
   Optional<Map<String, RateLimit>> getRatelimits(Type type) {
-    if (rateLimits != null) {
-      return Optional.ofNullable(rateLimits.row(type));
-    }
-    return Optional.empty();
+    return Optional.ofNullable(rateLimits).map(limits -> limits.row(type));
   }
 }