Deprecate empty index name

[morozov]

Q	A
Type	deprecation

Of all supported platforms, only PostgreSQL allows to omit the name in the CREATE INDEX statement (it will generate it internally). The rest of the platform require the index name to be defined as part of their syntax.

Handling of the null value of the $name name parameter of the Index constructor was added in #2854. Quite likely, it was done in order to satisfy a test for the code that would use an index to create a primary key constraint. Thus, there was a false assumption that given that constraints can be unnamed, indexes should be as well (see #2854 (comment)).

Unlike foreign key and unique constraints, for which DBAL generates the name, if it's omitted in the definition, it doesn't do that for indexes, so two indexes with empty name cannot even co-exist in a table definition:

new Table('t', [
    new Column('c1', Type::getType('integer')),
    new Column('c2', Type::getType('integer')),
], [
    new Index(null, ['c1']),
    new Index(null, ['c2']),
]);

// Doctrine\DBAL\Schema\Exception\IndexAlreadyExists:
//   An index with name "" was already defined on table "t".