Handle BUNDLER_VERSION being set to an empty string

[ccutrer]

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.

What was the end-user or developer problem that led to this PR?

I have a Dockerfile that has an upstream Dockerfile that sets BUNDLER_VERSION, but I don't want that set. I instead want bundler to automatically use the correct version from my Gemfile.lock. Unfortunately, once an env var is set in Docker, it can't be unset. Only overridden (and set to the empty string). When I do that, I get this:

% BUNDLER_VERSION= bundle --version                                
/Users/cody/.rubies/ruby-3.1.3/lib/ruby/site_ruby/3.1.0/rubygems/requirement.rb:107:in `parse': Illformed requirement [""] (Gem::Requirement::BadRequirementError)
	from /Users/cody/.rubies/ruby-3.1.3/lib/ruby/site_ruby/3.1.0/rubygems/requirement.rb:139:in `block in initialize'
	from /Users/cody/.rubies/ruby-3.1.3/lib/ruby/site_ruby/3.1.0/rubygems/requirement.rb:139:in `map!'
	from /Users/cody/.rubies/ruby-3.1.3/lib/ruby/site_ruby/3.1.0/rubygems/requirement.rb:139:in `initialize'
	from /Users/cody/src/canvas-lms/bin/bundle:114:in `new'
	from /Users/cody/src/canvas-lms/bin/bundle:114:in `activate_bundler'
	from /Users/cody/src/canvas-lms/bin/bundle:102:in `load_bundler!'
	from /Users/cody/src/canvas-lms/bin/bundle:128:in `<main>'

What is your fix for the problem, implemented in this PR?

Treat an empty BUNDLER_VERSION env var the same as not present at all.

Make sure the following tasks are checked

• Describe the problem / feature
• Write tests for features and bug fixes
• Write code to solve the problem
• Make sure you follow the current code style and write meaningful commit messages without tags

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[segiddins]

we are now 3.0+, this line should be deletable

[simi]

Instead of trying to update all usage of ENV["BUNDLER_VERSION"], wouldn't make sense to do the opposite and update the ENV["BUNDLER_VERSION"] to nil if set to empty string somewhere in initialisation code? That way it will not be possible to forget handling "" as nil in the future.