fix(ruby): Make custom instrumentation samples nil safe (#16554)

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## PRE-MERGE CHECKLIST

*Make sure you've checked the following before merging your changes:*

- [ ] Checked Vercel preview for correctness, including links
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Author: sl0thentr0py

platform-includes/performance/add-spans-example/ruby.mdx

@@ -8,7 +8,7 @@ class OrdersController < ApplicationController
     order = Order.new
 
     Sentry.with_child_span(op: :process_items, description: 'processing items') do |span|
-      span.set_data(:key, 'value')
+      span&.set_data(:key, 'value')
 
       order.process_items(params)
     end
@@ -28,12 +28,14 @@ class OrdersController < ApplicationController
     order = Order.new
     transaction = Sentry.get_current_scope.get_transaction
 
-    transaction.with_child_span(op: :process_items, description: 'processing items') do |span|
-      span.set_data(:key, 'value')
+    if transaction
+      transaction.with_child_span(op: :process_items, description: 'processing items') do |span|
+        span&.set_data(:key, 'value')
+        order.process_items(params)
+      end # the child span ends with the block
+    else
       order.process_items(params)
-    end # the child span ends with the block
+    end
   end
 end
 ```
-
-Keep in mind that there may not be an active transaction, in which case `get_transaction` returns `nil`. This case needs to be handled manually and is missing from this example.

platform-includes/performance/improving-data/ruby.mdx

@@ -22,6 +22,8 @@ You can add data attributes to your spans the same way, with the same type restr
 
 ```ruby
 Sentry.with_child_span(op: "my-span") do |span|
+  next unless span
+
   span.set_data("my-data-attribute-1", "value1")
   span.set_data("my-data-attribute-2", 42)
   span.set_data("my-data-attribute-3", true)

platform-includes/performance/retrieve-transaction/ruby.mdx

@@ -7,7 +7,7 @@ transaction = Sentry.get_current_scope.get_transaction || Sentry.start_transacti
 
 span = transaction.start_child(op: 'operation')
 # perform the operation
-span.finish
+span.finish if span
 ```
 
 ## Retrieve the Current Span