Null safety cleanup

Author: hazendaz

src/main/java/org/apache/ibatis/jdbc/AbstractSQL.java

@@ -712,7 +712,7 @@ private void sqlClause(SafeAppendable builder, String keyword, List<String> part
         String last = "________";
         for (int i = 0, n = parts.size(); i < n; i++) {
           String part = parts.get(i);
-          if (i > 0 && !part.equals(AND) && !part.equals(OR) && !last.equals(AND) && !last.equals(OR)) {
+          if (i > 0 && !AND.equals(part) && !OR.equals(part) && !AND.equals(last) && !OR.equals(last)) {
             builder.append(conjunction);
           }
           builder.append(part);