Handle nil in add_index :length option in MySQL

Our schema.rb is being generated with an `add_index` line similar to this:

    add_index "foo", ["foo", "bar"], :name => "xxx", :length => {"foo"=>8, "bar=>nil}

This is the same as it was on Rails 3.1.3, however, now when that
schema.rb is evaluated, its generating bad SQL in MySQL:

    Mysql::Error: You have an error in your SQL syntax; check the manual
    that corresponds to your MySQL server version for the right syntax
    to use near '))' at line 1: CREATE UNIQUE INDEX
    `xxx` ON `foo` (`foo`(8), `bar`())

This commit adds a check for nil on the length attribute to prevent the
empty parens from being output.

0 files changed, 0 insertions, 0 deletions