fix: Prevent usage of Oj once requiring it already raised an exception

Fixes the following exception, which happens,
when the require statement has raised an exception,
but only after the 'Oj' name is already defined.

````
E, [2024-04-29T13:33:24.549280 #108] ERROR -- : undefined method `dump' for Oj:Module

        defined?(Oj) ? Oj.dump(data, mode: :strict) : JSON.dump(data)
                         ^^^^^
Did you mean?  dup
```

Signed-off-by: Ferenc Géczi <[email protected]>

Author: Ferenc-

lib/instana/backend/request_client.rb

@@ -8,8 +8,10 @@
 # :nocov:
 begin
   require 'oj'
+  INSTANA_USE_OJ = true
 rescue LoadError => _e
   Instana.logger.warn("Unable to load Oj.")
+  INSTANA_USE_OJ = false
 end
 # :nocov:
 
@@ -66,7 +68,7 @@ def send_request(method, path, data = nil, headers = {})
 
       def encode_body(data)
         # :nocov:
-        defined?(Oj) ? Oj.dump(data, mode: :strict) : JSON.dump(data)
+        INSTANA_USE_OJ ? Oj.dump(data, mode: :strict) : JSON.dump(data)
         # :nocov:
       end
     end