Make sure to pass in a string

Not all RDBMS have a concept of database, apparently (see SQLAnywhere).
Those who do not should pass in a string to stay compatible with the
signature.

lib/Doctrine/DBAL/Platforms/AbstractPlatform.php

@@ -2817,7 +2817,7 @@ abstract class AbstractPlatform
 
     /**
      * @param string $table
-     * @param string|null $database
+     * @param string $database
      *
      * @return string
      *

tests/Doctrine/Tests/DBAL/Platforms/SQLAnywhere12PlatformTest.php

@@ -52,7 +52,7 @@ class SQLAnywhere12PlatformTest extends SQLAnywhere11PlatformTest
         );
         self::assertEquals(
             'SELECT sequence_name, increment_by, start_with, min_value FROM SYS.SYSSEQUENCE',
-            $this->platform->getListSequencesSQL(null)
+            $this->platform->getListSequencesSQL('')
         );
     }