avoid a warning in test suite (#4687)

2025-05-23T20:57:17.7367220Z /Users/runner/work/dd-trace-rb/dd-trace-rb/spec/spec_helper.rb:174: warning: instance variable @caller not initialized

Co-authored-by: Oleg Pudeyev <[email protected]>

Author: p-datadog

spec/spec_helper.rb

@@ -171,7 +171,7 @@
       # Exclude acceptable background threads
       background_threads = Thread.list.reject do |t|
         group_name = t.group.instance_variable_get(:@group_name) if t.group.instance_variable_defined?(:@group_name)
-        caller = t.instance_variable_get(:@caller) || []
+        caller = t.instance_variable_defined?(:@caller) && t.instance_variable_get(:@caller) || []
         backtrace = t.backtrace || []
 
         # Current thread