Brace expansion discards empty alternatives

Created by: xTibor

Reproduction: This example comes from Rosetta Code:

$ echo It{{em,alic}iz,erat}e{d,}

Expected behavior:

Itemized Itemize Italicized Italicize Iterated Iterate

Actual behavior:

Itemized Italicized Iterated

Build information: rustc -V: rustc 1.28.0-nightly (5d0631a64 2018-05-30) git rev-parse HEAD: 91598eac System: KDE neon 5.12 (Ubuntu 16.04)