Added test for ALTERing Nullable columns [#CLICKHOUSE-4].

dbms/tests/queries/0_stateless/00456_alter_nullable.reference

+Hello
+World
+Hello
+World
+\N
+Hello
+World
+xyz
+\N
+Hello
+World
+xyz\0\0

dbms/tests/queries/0_stateless/00456_alter_nullable.sql

+DROP TABLE IF EXISTS test.nullable_alter;
+CREATE TABLE test.nullable_alter (d Date DEFAULT '2000-01-01', x String) ENGINE = MergeTree(d, d, 1);
+
+INSERT INTO test.nullable_alter (x) VALUES ('Hello'), ('World');
+SELECT x FROM test.nullable_alter ORDER BY x;
+
+ALTER TABLE test.nullable_alter MODIFY COLUMN x Nullable(String);
+SELECT x FROM test.nullable_alter ORDER BY x;
+
+INSERT INTO test.nullable_alter (x) VALUES ('xyz'), (NULL);
+SELECT x FROM test.nullable_alter ORDER BY x NULLS FIRST;
+
+ALTER TABLE test.nullable_alter MODIFY COLUMN x Nullable(FixedString(5));
+SELECT x FROM test.nullable_alter ORDER BY x NULLS FIRST;
+
+DROP TABLE test.nullable_alter;