fix(mtr): Add test cases for NULL value select on UNIQUE constraint(#281) (#379)

* fix(core): Double prepare on lex unit for cts with union(#226)

fix(core): MTR + Double prepare on lex unit for ctas with union(#226)

* fix(mtr): Add test cases for NULL value select on UNIQUE constraint(#281)
Co-authored-by: Nmergify[bot] <37929162+mergify[bot]@users.noreply.github.com>

mysql-test/suite/tianmu/r/issue281.result

+use test;
+CREATE TABLE t1 (f1 INT UNIQUE) ENGINE=TIANMU;
+INSERT INTO t1 VALUES(1),(2),(NULL),(NULL);
+SELECT * FROM t1 WHERE f1 IS NULL;
+f1
+NULL
+NULL
+SELECT * FROM t1 WHERE f1 IS NOT NULL;
+f1
+1
+2
+SELECT * FROM t1;
+f1
+1
+2
+NULL
+NULL
+DROP TABLE t1;

mysql-test/suite/tianmu/t/issue281.test

+use test;
+CREATE TABLE t1 (f1 INT UNIQUE) ENGINE=TIANMU;
+INSERT INTO t1 VALUES(1),(2),(NULL),(NULL);
+SELECT * FROM t1 WHERE f1 IS NULL;
+SELECT * FROM t1 WHERE f1 IS NOT NULL;
+SELECT * FROM t1;
+DROP TABLE t1;