Handle BUNDLER_VERSION being set to an empty string

This is useful, in case you're using Docker, and an upstream
Dockerfile sets BUNDLER_VERSION to something you don't want.
It's impossible to unset it... only override to be the empty
string.

bundler/lib/bundler/self_manager.rb

@@ -90,7 +90,7 @@ def needs_switching?
     end
 
     def autoswitching_applies?
-      ENV["BUNDLER_VERSION"].nil? &&
+      (ENV["BUNDLER_VERSION"].nil? || ENV["BUNDLER_VERSION"].empty?) &&
         Bundler.rubygems.supports_bundler_trampolining? &&
         SharedHelpers.in_bundle? &&
         lockfile_version

bundler/lib/bundler/templates/Executable.bundler

@@ -18,7 +18,7 @@ m = Module.new do
   end
 
   def env_var_version
-    ENV["BUNDLER_VERSION"]
+    ENV["BUNDLER_VERSION"]&.then { |v| v.empty? ? nil : v }
   end
 
   def cli_arg_version

bundler/spec/commands/binstubs_spec.rb

@@ -150,6 +150,10 @@
           sys_exec "bin/bundle install", :env => { "BUNDLER_VERSION" => "999.999.998", "DEBUG" => "1" }, :raise_on_error => false
           expect(out).to include %(Using bundler 999.999.998\n)
         end
+
+        it "runs correctly even if empty" do
+          sys_exec "bin/bundle install", :env => { "BUNDLER_VERSION" => "" }
+        end
       end
 
       context "when a lockfile exists with a locked bundler version" do

lib/rubygems/bundler_version_finder.rb

@@ -3,6 +3,7 @@
 module Gem::BundlerVersionFinder
   def self.bundler_version
     v = ENV["BUNDLER_VERSION"]
+    v = nil if v&.empty?
 
     v ||= bundle_update_bundler_version
     return if v == true