hush: fix a few more corner cases with empty-expanding cmds

See added testcases

function                                             old     new   delta
run_pipe                                            1723    1784     +61

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>

shell/hush_test/hush-redir/redir_backquote1.right

@@ -0,0 +1,11 @@
+hush: can't open '/cant/be/created': No such file or directory
+First
+One:1 v1:[]
+hush: can't open '/cant/be/created': No such file or directory
+Second
+One:1 v2:[]
+Third
+Zero:0 v3:[]
+Fourth
+Zero:0 v4:[]
+Zero:0 v5:[1]

shell/hush_test/hush-redir/redir_backquote1.tests

@@ -0,0 +1,19 @@
+v=v
+v=`echo First >&2` `` >/cant/be/created
+echo One:$? v1:"[$v]"
+
+v=v
+v=`echo Second >&2` `true` >/cant/be/created
+echo One:$? v2:"[$v]"
+
+v=v
+v=`echo Third >&2` `true` 2>/dev/null
+echo Zero:$? v3:"[$v]"
+
+v=v
+v=`echo Fourth >&2` `false` 2>/dev/null
+echo Zero:$? v4:"[$v]"
+
+v=v
+v=`echo $?` `false` 2>/dev/null
+echo Zero:$? v5:"[$v]"