hush: fix handling of empty arguments

function                                             old     new   delta
builtin_exec                                          25      83     +58
parse_stream                                        2242    2261     +19
run_pipe                                            1782    1787      +5
static.pseudo_null_str                                 -       3      +3
------------------------------------------------------------------------------
(add/remove: 1/0 grow/shrink: 3/0 up/down: 85/0)               Total: 85 bytes

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>
diff --git a/shell/hush_test/hush-psubst/emptytick.right b/shell/hush_test/hush-psubst/emptytick.right
index d4b70c5..1f60ecf 100644
--- a/shell/hush_test/hush-psubst/emptytick.right
+++ b/shell/hush_test/hush-psubst/emptytick.right
@@ -1,14 +1,17 @@
 0
 0
+hush: can't execute '': No such file or directory
+0
+hush: can't execute '': No such file or directory
 0
 0
 0
 0
 0
+hush: can't execute '': No such file or directory
+0
+hush: can't execute '': No such file or directory
 0
 0
 0
-0
-0
-0
-0
+hush: can't execute '': No such file or directory