Update gitlab-experiment to 0.4.5

This makes two functionally similar but helpful improvements in terms of
more clarity and better behavior when circumventing existing codepaths.

Gemfile

@@ -478,7 +478,7 @@ gem 'flipper', '~> 0.17.1'
 gem 'flipper-active_record', '~> 0.17.1'
 gem 'flipper-active_support_cache_store', '~> 0.17.1'
 gem 'unleash', '~> 0.1.5'
-gem 'gitlab-experiment', '~> 0.4.4'
+gem 'gitlab-experiment', '~> 0.4.5'
 
 # Structured logging
 gem 'lograge', '~> 0.5'

Gemfile.lock

@@ -422,7 +422,7 @@ GEM
     github-markup (1.7.0)
     gitlab-chronic (0.10.5)
       numerizer (~> 0.2)
-    gitlab-experiment (0.4.4)
+    gitlab-experiment (0.4.5)
       activesupport (>= 3.0)
       scientist (~> 1.5, >= 1.5.0)
     gitlab-fog-azure-rm (1.0.0)
@@ -1361,7 +1361,7 @@ DEPENDENCIES
   gitaly (~> 13.7.0.pre.rc1)
   github-markup (~> 1.7.0)
   gitlab-chronic (~> 0.10.5)
-  gitlab-experiment (~> 0.4.4)
+  gitlab-experiment (~> 0.4.5)
   gitlab-fog-azure-rm (~> 1.0)
   gitlab-labkit (= 0.14.0)
   gitlab-license (~> 1.0)

app/experiments/application_experiment.rb

@@ -19,7 +19,9 @@ class ApplicationExperiment < Gitlab::Experiment
   private
 
   def resolve_variant_name
-    variant_names.first if Feature.enabled?(name, self, type: :experiment)
+    return variant_names.first if Feature.enabled?(name, self, type: :experiment)
+
+    nil # Returning nil vs. :control is important for not caching and rollouts.
   end
 
   # Cache is an implementation on top of Gitlab::Redis::SharedState that also
@@ -61,7 +63,7 @@ class ApplicationExperiment < Gitlab::Experiment
     end
 
     def hkey(key)
-      key.split(':') # this assumes the default strategy in gitlab-experiment
+      key.to_s.split(':') # this assumes the default strategy in gitlab-experiment
     end
 
     def read_entry(key, **options)