fix: MariaDB VIRTUAL + [NOT NULL|NULL] error (#7022)

* Fix MariaDB VIRTUAL [NOT NULL|NULL] error

Fixes #2691

query failed: ALTER TABLE `customer` CHANGE `fullName` `fullName` varchar(255) AS (CONCAT(`firstName`, ' ', `lastName`)) VIRTUAL NULL
error: Error: You have an error in your SQL syntax; check the manual that corresponds to your MariaDB server version for the right syntax to use near 'NULL' at line 1

* slightly change code style to improve readability

Co-authored-by: Umed Khudoiberdiev <pleerock.me@gmail.com>

src/driver/mysql/MysqlQueryRunner.ts

@@ -1857,10 +1857,17 @@ export class MysqlQueryRunner extends BaseQueryRunner implements QueryRunner {
             c += ` CHARACTER SET "${column.charset}"`;
         if (column.collation)
             c += ` COLLATE "${column.collation}"`;
-        if (!column.isNullable)
-            c += " NOT NULL";
-        if (column.isNullable)
-            c += " NULL";
+
+        const isMariaDb = this.driver.options.type === "mariadb";
+        if (isMariaDb && column.asExpression && (column.generatedType || "VIRTUAL") === "VIRTUAL") {
+            // do nothing - MariaDB does not support NULL/NOT NULL expressions for VIRTUAL columns
+        } else {   
+            if (!column.isNullable)
+                c += " NOT NULL";
+            if (column.isNullable)
+                c += " NULL";
+        }
+
         if (column.isPrimary && !skipPrimary)
             c += " PRIMARY KEY";
         if (column.isGenerated && column.generationStrategy === "increment") // don't use skipPrimary here since updates can update already exist primary without auto inc.