renameColumn: non-null column can't default null

This fixes a bug in `renameColumn()` of migration. If one tries to rename a column which was originally `NOT NULL` and doesn't have a default value set, it will incorrectly try to set the default value as `NULL` after renaming.

Code explanation:

```javascript
// query-interface.js L391
      this.describeTable(tableName).success(function(data) {
        /*
         * describeTable() returns null for defaultValue,
         * even when no defaultValue is set
         */

        data = data[attrNameBefore] || {}

        var options =  {}

        options[attrNameAfter] = {
          attribute:    attrNameAfter,
          type:         data.type,
          allowNull:    data.allowNull,
          defaultValue: data.defaultValue
        }

        // fix: a not-null column cannot have null as default value
        if (data.defaultValue === null && !data.allowNull) {
          delete options[attrNameAfter].defaultValue;
        }
```