Pathname#abv: handle the case `du` returns empty string

This can happen when read permission is denied.

Fixes Homebrew/homebrew#41925.

Closes Homebrew/homebrew#42011.

Signed-off-by: Xu Cheng <xucheng@me.com>

Library/Homebrew/extend/pathname.rb

@@ -393,7 +393,9 @@ class Pathname
     out = ""
     n = Utils.popen_read("find", expand_path.to_s, "-type", "f", "!", "-name", ".DS_Store").split("\n").size
     out << "#{n} files, " if n > 1
-    out << Utils.popen_read("/usr/bin/du", "-hs", expand_path.to_s).split("\t")[0].strip
+    size = Utils.popen_read("/usr/bin/du", "-hs", expand_path.to_s).split("\t")[0]
+    size ||= "0B"
+    out << size.strip
     out
   end