cleanup StringUtils.cache

1. No need to call `hashCode` if  `cache=null`
2. Remove redundant `null` comparison. `String.equals` will anyway return `false`

h2/src/main/org/h2/util/StringUtils.java

@@ -941,16 +941,14 @@ public class StringUtils {
         } else if (s.isEmpty()) {
             return "";
         }
-        int hash = s.hashCode();
         String[] cache = getCache();
         if (cache != null) {
+            int hash = s.hashCode();
             int index = hash & (SysProperties.OBJECT_CACHE_SIZE - 1);
             String cached = cache[index];
-            if (cached != null) {
             if (s.equals(cached)) {
                 return cached;
             }
-            }
             cache[index] = s;
         }
         return s;