HBASE-23375 HBASE-22441 BucketCache NullPointerException in cacheBlock

branches-1 backport. closes #907

Co-authored-by: binlijin <binlijin@apache.org>
Signed-off-by: binlijin <binlijin@apache.org>
Signed-off-by: Viraj Jasani <virajjasani007@gmail.com>
Signed-off-by: Sean Busbey <busbey@apache.org>

hbase-server/src/main/java/org/apache/hadoop/hbase/io/hfile/BlockCacheUtil.java

@@ -230,6 +230,9 @@ public static int validateBlockAddition(Cacheable existing, Cacheable newBlock,
   public static boolean shouldReplaceExistingCacheBlock(BlockCache blockCache,
       BlockCacheKey cacheKey, Cacheable newBlock) {
     Cacheable existingBlock = blockCache.getBlock(cacheKey, false, false, false);
+    if (existingBlock == null) {
+      return true;
+    }
     int comparison = BlockCacheUtil.validateBlockAddition(existingBlock, newBlock, cacheKey);
     if (comparison < 0) {
       LOG.warn("Cached block contents differ by nextBlockOnDiskSize, the new block has "