Fix warning during ruby_version specs

The following warning:

```
/path/to/spec/bundler/ruby_version_spec.rb:404: warning: instance variable @ruby_version not initialized
```

1 files changed, 14 insertions, 6 deletions

diff --git a/spec/bundler/ruby_version_spec.rb b/spec/bundler/ruby_version_spec.rb
index 868d81088d..8c6c071d7f 100644
--- a/spec/bundler/ruby_version_spec.rb
+++ b/spec/bundler/ruby_version_spec.rb
@@ -400,12 +400,20 @@ RSpec.describe "Bundler::RubyVersion and its subclasses" do
       let(:bundler_system_ruby_version) { subject }
 
       around do |example|
-        begin
-          old_ruby_version = Bundler::RubyVersion.instance_variable_get("@ruby_version")
-          Bundler::RubyVersion.instance_variable_set("@ruby_version", nil)
-          example.run
-        ensure
-          Bundler::RubyVersion.instance_variable_set("@ruby_version", old_ruby_version)
+        if Bundler::RubyVersion.instance_variable_defined?("@ruby_version")
+          begin
+            old_ruby_version = Bundler::RubyVersion.instance_variable_get("@ruby_version")
+            Bundler::RubyVersion.remove_instance_variable("@ruby_version")
+            example.run
+          ensure
+            Bundler::RubyVersion.instance_variable_set("@ruby_version", old_ruby_version)
+          end
+        else
+          begin
+            example.run
+          ensure
+            Bundler::RubyVersion.remove_instance_variable("@ruby_version")
+          end
         end
       end