hush: fix handling of '' in ${var:+ARG}

This wasn't an ash bug in dollar_altvalue9, it was hush bug (and bash!)

function                                             old     new   delta
expand_one_var                                      2236    2254     +18
expand_vars_to_list                                 1097    1103      +6
------------------------------------------------------------------------------
(add/remove: 0/0 grow/shrink: 2/0 up/down: 24/0)               Total: 24 bytes

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

shell/ash_test/ash-quoting/dollar_altvalue2.right

@@ -0,0 +1,69 @@
+Unquoted '':
+start:
+||
+end
+start:
+||
+end
+start:
+||
+end
+start:
+||
+end
+start:
+||
+||
+end
+
+Unquoted "":
+start:
+||
+end
+start:
+||
+end
+start:
+||
+end
+start:
+||
+end
+start:
+||
+||
+end
+
+Quoted '':
+start:
+|''|
+end
+start:
+|'' |
+end
+start:
+| ''|
+end
+start:
+| '' |
+end
+start:
+|'' ''|
+end
+
+Quoted "":
+start:
+||
+end
+start:
+| |
+end
+start:
+| |
+end
+start:
+|  |
+end
+start:
+| |
+end