regression when parent is Pathname and it is empty

[znz]

Dir.mktmpdir with Pathname works before Ruby 3.3.
But tmpdir of ruby 3.4.0dev failed when path is empty.

When Pathname#empty? returns false, Dir.mktmpdir raises ArgumentError.

% ruby -r pathname -r tmpdir -e 'Dir.mktmpdir{|outer|n=Pathname(outer);(n+"foo").open("w");Dir.mktmpdir("prefix-", outer){}}'
% ruby -r pathname -r tmpdir -e 'Dir.mktmpdir{outer=Pathname(_1);Dir.mktmpdir("prefix-", outer){}} rescue p $!'
#<ArgumentError: empty parent path>
% ruby -v
ruby 3.4.0dev (2024-12-16T05:44:05Z master 7c2660b347) +PRISM [x86_64-linux]