HBASE-22520 Avoid possible NPE while performing seekBefore in Hal… (apache#281)

HBASE-22520 Avoid possible NPE while performing seekBefore in HalfStoreFileReader

(cherry picked from commit 1904929)

Change-Id: Id793c6eb9d542e423dccb313ce74f460f4e97432

Author: virajjasani

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

@@ -60,9 +60,9 @@ public class HalfStoreFileReader extends StoreFileReader {
   // i.e. empty column and a timestamp of LATEST_TIMESTAMP.
   protected final byte [] splitkey;
 
-  protected final Cell splitCell;
+  private final Cell splitCell;
 
-  private Optional<Cell> firstKey = null;
+  private Optional<Cell> firstKey = Optional.empty();
 
   private boolean firstKeySeeked = false;
 
@@ -269,7 +269,8 @@ public int reseekTo(Cell key) throws IOException {
       public boolean seekBefore(Cell key) throws IOException {
         if (top) {
           Optional<Cell> fk = getFirstKey();
-          if (PrivateCellUtil.compareKeyIgnoresMvcc(getComparator(), key, fk.get()) <= 0) {
+          if (fk.isPresent() &&
+                  PrivateCellUtil.compareKeyIgnoresMvcc(getComparator(), key, fk.get()) <= 0) {
             return false;
           }
         } else {