[dcl.init.aggr]p9 says this:

An array of unknown bound initialized with a brace-enclosed initializer-list containing n initializer-clauses, where n shall be greater than zero, is defined as having n elements.

The "where n shall be greater than zero" is unnecessary, as an initializer-list is never empty, and as such, a brace-enclosed initialize-list always has at least one element.

The restriction is also mentioned a bit later:

An empty initializer list {} shall not be used as the initializer-clause for an array of unknown bound.

I don't quite understand it (what initializer-clause?), but it seems to also disallow int a[] = {};.