Fix creation of unnamed unique constraints

[morozov]

Q	A
Type	bug

Note that even though the $name parameter in the UniqueConstraint constructor isn't nullable, the original implementation attempted to handle constraints with empty name:

dbal/lib/Doctrine/DBAL/Platforms/AbstractPlatform.php

Lines 2396 to 2397 in b0cc669

	$constraintName = $name->getQuotedName($this);
	$constraintName = ! empty($constraintName) ? $constraintName . ' ' : '';

Also, constraint names in SQL are optional for all supported platforms. Therefore, I consider the behavior of improper handling of unnamed constraints a bug.

Two issues are being fixed:

1. If the unique constraint name is omitted (empty), the DBAL will generate invalid DDL (both in the case of CREATE TABLE and ALTER TABLE).
2. If there are more than one unique constraint with omitted name, their definitions will clash and only one will be created.

Both of these edge cases are correctly handled for foreign key constraints, which can be referenced as the proper implementation.

[greg0ire]

Didn't know about this syntax :)