[DBAL-789] Fix default values for TEXT/BLOB column type on MySQL

MySQL does not support default values for `TEXT` and `BLOB` type columns. This causes `CREATE TABLE` statements to fail if a default value is supplied for those column types:

```sql
Doctrine\DBAL\DBALException: An exception occurred while executing 'CREATE TABLE text_blob_default_value (def_text LONGTEXT DEFAULT 'def' NOT NULL, def_text_null LONGTEXT DEFAULT 'def', def_blob LONGBLOB DEFAULT 'def' NOT NULL, def_blob_null LONGBLOB DEFAULT 'def') DEFAULT CHARACTER SET utf8 COLLATE utf8_unicode_ci ENGINE = InnoDB':

SQLSTATE[42000]: Syntax error or access violation: 1101 BLOB/TEXT column 'def_text' can't have a default value
```

Additionally useless `ALTER TABLE` statements are created for those mappings with the schema tool because online and offline schema differ for those column types concerning default values:

```sql
ALTER TABLE creative CHANGE filename filename LONGTEXT DEFAULT '' NOT NULL;
```