Refactor `join("")` to `join`.

Library/Homebrew/rubocops/extend/formula_cop.rb

@@ -460,7 +460,7 @@ module RuboCop
         when :str
           node.str_content
         when :dstr
-          node.each_child_node(:str).map(&:str_content).join("")
+          node.each_child_node(:str).map(&:str_content).join
         when :const
           node.const_name
         when :sym

Library/Homebrew/test/cask/download_strategy_spec.rb

@@ -148,7 +148,7 @@ describe "download strategies", :cask do
           "DBw2KdR24q9t1wfjS9LUzelf5TWk6ojj8p9%2FHjl%2Fi%2FVCXN",
           "N4o1mW%2FMayy2tTY1qcC%2FTmqI1ulZS8SNuaSgr9Iys9oDF1%2",
           "BPK%2B4Sg==",
-        ].join("")
+        ].join
       end
 
       describe "#tarball_path" do

Library/Homebrew/test/cask/dsl_spec.rb

@@ -20,7 +20,7 @@ describe Hbc::DSL, :cask do
     }
 
     it "prints a warning that it has encountered an unexpected method" do
-      expected = Regexp.compile(<<~EOS.lines.map(&:chomp).join(""))
+      expected = Regexp.compile(<<~EOS.lines.map(&:chomp).join)
         (?m)
         Warning:
         .*

Library/Homebrew/test/cask/pkg_spec.rb

@@ -134,7 +134,7 @@ describe Hbc::Pkg, :cask do
           <string>#{volume}</string>
           <key>paths</key>
           <dict>
-            #{(pkg_files + pkg_directories).map { |f| "<key>#{f}</key><dict></dict>" }.join("")}
+            #{(pkg_files + pkg_directories).map { |f| "<key>#{f}</key><dict></dict>" }.join}
           </dict>
         </dict>
         </plist>

Library/Homebrew/test/cask/system_command_spec.rb

@@ -109,7 +109,7 @@ describe Hbc::SystemCommand, :cask do
       end
 
       it "echoes only STDOUT" do
-        expected = [1, 3, 5].map { |i| "#{i}\n" }.join("")
+        expected = [1, 3, 5].map { |i| "#{i}\n" }.join
         expect {
           described_class.run(command, options)
         }.to output(expected).to_stdout