Fix the window-function group_concat() so that it returns an empty string

Fix the window-function group_concat() so that it returns an empty string
if it has one or more empty string inputs.  This fixes
a bug introduced by [c6da39115d3e2b0f] on 2019-03-26 (version 3.28.0) and
reported by [forum/forumpost/bf8f43aa522c2299|forum post bf8f43aa522c2299].

FossilOrigin-Name: cec6bb3fc9932ea78ec8e63d9c2d4e56a4d94b8973b9ea46033cc4baa87c0476