[rubygems/rubygems] Leave ":" after MANPATH when not set

So that system man pages still work after a gem with man pages overrides
it.

https://github.com/rubygems/rubygems/commit/1031879b87

lib/bundler/runtime.rb

@@ -265,7 +265,7 @@ module Bundler
 
       return if manuals.empty?
       Bundler::SharedHelpers.set_env "MANPATH", manuals.concat(
-        ENV["MANPATH"].to_s.split(File::PATH_SEPARATOR)
+        ENV["MANPATH"] ? ENV["MANPATH"].to_s.split(File::PATH_SEPARATOR) : [""]
       ).uniq.join(File::PATH_SEPARATOR)
     end
 

spec/bundler/runtime/setup_spec.rb

@@ -745,19 +745,16 @@ end
     expect(err).to be_empty
   end
 
-  describe "$MANPATH" do
-    before do
+  context "when the user has `MANPATH` set", :man do
+    before { ENV["MANPATH"] = "/foo#{File::PATH_SEPARATOR}" }
+
+    it "adds the gem's man dir to the MANPATH" do
       build_repo4 do
         build_gem "with_man" do |s|
           s.write("man/man1/page.1", "MANPAGE")
         end
       end
-    end
 
-    context "when the user has one set" do
-      before { ENV["MANPATH"] = "/foo#{File::PATH_SEPARATOR}" }
-
-      it "adds the gem's man dir to the MANPATH" do
       install_gemfile <<-G
         source "#{file_uri_for(gem_repo4)}"
         gem "with_man"
@@ -768,18 +765,48 @@ end
     end
   end
 
-    context "when the user does not have one set" do
+  context "when the user does not have `MANPATH` set", :man do
     before { ENV.delete("MANPATH") }
 
-      it "adds the gem's man dir to the MANPATH" do
+    it "adds the gem's man dir to the MANPATH, leaving : in the end so that system man pages still work" do
+      build_repo4 do
+        build_gem "with_man" do |s|
+          s.write("man/man1/page.1", "MANPAGE")
+        end
+
+        build_gem "with_man_overriding_system_man" do |s|
+          s.write("man/man1/ls.1", "LS MANPAGE")
+        end
+      end
+
       install_gemfile <<-G
         source "#{file_uri_for(gem_repo4)}"
         gem "with_man"
       G
 
-        run "puts ENV['MANPATH']"
-        expect(out).to eq(default_bundle_path("gems/with_man-1.0/man").to_s)
-      end
+      run <<~RUBY
+        puts ENV['MANPATH']
+        require "open3"
+        puts Open3.capture2e("man", "ls")[1].success?
+      RUBY
+
+      expect(out).to eq("#{default_bundle_path("gems/with_man-1.0/man")}#{File::PATH_SEPARATOR}\ntrue")
+
+      install_gemfile <<-G
+        source "#{file_uri_for(gem_repo4)}"
+        gem "with_man_overriding_system_man"
+      G
+
+      run <<~RUBY
+        puts ENV['MANPATH']
+        require "open3"
+        puts Open3.capture2e("man", "ls")[0]
+      RUBY
+
+      lines = out.split("\n")
+
+      expect(lines).to include("#{default_bundle_path("gems/with_man_overriding_system_man-1.0/man")}#{File::PATH_SEPARATOR}")
+      expect(lines).to include("LS MANPAGE")
     end
   end
 

spec/bundler/support/filters.rb

@@ -34,6 +34,7 @@ RSpec.configure do |config|
   config.filter_run_excluding :readline => Gem.win_platform?
   config.filter_run_excluding :jruby => RUBY_ENGINE != "jruby"
   config.filter_run_excluding :truffleruby => RUBY_ENGINE != "truffleruby"
+  config.filter_run_excluding :man => Gem.win_platform?
 
   config.filter_run_when_matching :focus unless ENV["CI"]
 end