fix(ruby): Make traces_sampler example safe (#11394)

platform-includes/performance/traces-sampler-as-sampler/ruby.mdx

@@ -9,7 +9,7 @@ Sentry.init do |config|
 
  # the sampling context also has the full rack environment if you want to check the path directly
  rack_env = sampling_context[:env]
- return 0.0 if rack_env['PATH_INFO'] =~ /health_check/
+ return 0.0 if rack_env && rack_env['PATH_INFO'] =~ /health_check/
 
  # transaction_context is the transaction object in hash form
  # keep in mind that sampling happens right after the transaction is initialized