Do not create volume when knapsack path is empty

lib/gitlab/qa/component/specs.rb

@@ -16,10 +16,12 @@ module Gitlab
           @docker = Docker::Engine.new(stream_output: true) # stream test output directly instead of through logger
           @env = {}
           @volumes = {}
-          @default_volumes = {
-            '/var/run/docker.sock' => '/var/run/docker.sock',
-            Runtime::Env.qa_knapsack_report_path => File.join(Docker::Volumes::QA_CONTAINER_WORKDIR, 'knapsack')
-          }.compact
+          @default_volumes = { '/var/run/docker.sock' => '/var/run/docker.sock' }
+          return if Runtime::Env.qa_knapsack_report_path.blank?
+
+          @default_volumes[Runtime::Env.qa_knapsack_report_path] = File.join(
+            Docker::Volumes::QA_CONTAINER_WORKDIR, 'knapsack'
+          )
         end
         def perform