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>
diff --git a/shell/hush_test/hush-psubst/falsetick2.right b/shell/hush_test/hush-psubst/falsetick2.right
new file mode 100644
index 0000000..670f560
--- /dev/null
+++ b/shell/hush_test/hush-psubst/falsetick2.right
@@ -0,0 +1 @@
+Two:2 v:[]