Empty ""s in ARG in ${x:+ARG} expand to no words instead of the empty word if prepended/appended with space

[Denys Vlasenko]

$ f() { for i; do echo "|$i|"; done; }
$ x=x
$ e=

$  f ${x:+""}
||
^^^^^^^^^^^ correct

$ f ${x:+ ""}
^^^^^^^^^^^ prints nothing, bug?

$  ${x:+"" }
^^^^^^^^^^^ prints nothing, bug?

$ f ${x:+"$e"}
||
^^^^^^^^^^^ correct

$ f ${x:+ "$e"}
^^^^^^^^^^^ prints nothing, bug?

$ f ${x:+"$e""$e"""}
||
^^^^^^^^^^^ correct

$ f ${x:+"$e""$e"""}
^^^^^^^^^^^ prints nothing, bug?


Similarly with: f ${x:+"`echo`" } etc...