That wasn't too difficult. See this commit. It will be available in version Text::CSV_XS version 1.18, but you can get it from github already.

$ perl -Mblib -MText::CSV_XS \
? -E'my $csv = Text::CSV_XS->new ({ quote_empty => 1 });' \
? -E'$csv->combine (1, undef, "", " ", 2);' \
? -E'say $csv->string'
1,,""," ",2
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update I forgot I already implemented this in the perl6 version Text::CSV.