hush: fix word splitting in ${v:+ARG} - dollar_altvalue1 test

hush: fix word splitting in ${v:+ARG} - dollar_altvalue1 test

ash might be a bit buggy, need to investigate dollar_altvalue9 test

function                                             old     new   delta
expand_one_var                                      1639    2236    +597
expand_variables                                     112     128     +16
expand_vars_to_list                                 1117    1097     -20
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(add/remove: 0/0 grow/shrink: 2/1 up/down: 613/-20)           Total: 593 bytes

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