hush: speed up ${var:+ARG} for literal ARGs

hush: speed up ${var:+ARG} for literal ARGs

function                                             old     new   delta
first_special_char_in_vararg                           -      52     +52
expand_one_var                                      2248    2296     +48
encode_then_expand_vararg                            357     336     -21
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(add/remove: 1/0 grow/shrink: 1/1 up/down: 100/-21)            Total: 79 bytes

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