Return empty log correlation when tracing is disabled

[marcotc]

If you completely disabled tracing, you will see no spans in the Datadog app, but your logs will still look like this:

2024-06-14 16:01:39 D, [2024-06-14T20:01:39.579237 #1] DEBUG -- : [dd.env=test dd.service=my-sev dd.version=1.0 dd.trace_id=123123443424 dd.span_id=0 ddsource=ruby]   Note Load (0.3ms)  SELECT  "tests".* FROM "tests" WHERE "tests"."id" = $1 LIMIT $2  [["id", 1], ["LIMIT", 1]]
2024-06-14 16:01:39 D, [2024-06-14T20:01:39.579349 #1] DEBUG -- : [dd.env=test dd.service=my-sev dd.version=1.0 dd.trace_id=123123443424 dd.span_id=0 ddsource=ruby]   ↳ app/controllers/tests_controller.rb:123
2024-06-14 16:01:39 I, [2024-06-14T20:01:39.580459 #1]  INFO -- : [dd.env=test dd.service=my-sev dd.version=1.0 dd.trace_id=123123443424 dd.span_id=0 ddsource=ruby] Completed 500 Internal Server Error in 10ms (ActiveRecord: 3.1ms)
2024-06-14 16:01:39 F, [2024-06-14T20:01:39.580794 #1] FATAL -- : [dd.env=test dd.service=my-sev dd.version=1.0 dd.trace_id=123123443424 dd.span_id=0 ddsource=ruby]   
2024-06-14 16:01:39 F, [2024-06-14T20:01:39.580823 #1] FATAL -- : [dd.env=test dd.service=my-sev dd.version=1.0 dd.trace_id=123123443424 dd.span_id=0 ddsource=ruby] ActiveRecord::RecordNotFound (Couldn't find Note with 'id'=1):
2024-06-14 16:01:39 F, [2024-06-14T20:01:39.580857 #1] FATAL -- : [dd.env=test dd.service=my-sev dd.version=1.0 dd.trace_id=123123443424 dd.span_id=0 ddsource=ruby]   
2024-06-14 16:01:39 F, [2024-06-14T20:01:39.580872 #1] FATAL -- : [dd.env=test dd.service=my-sev dd.version=1.0 dd.trace_id=123123443424 dd.span_id=0 ddsource=ruby] app/controllers/test_controller.rb:123:in `show'

This information pollutes the logs, it is useless without tracing enabled.

In particular, it is common to disable tracing in development environments. This issue makes logs in such environments much less developer friendly.

This PR returns a fallback empty String for log_correlation when tracing is disabled.
Because the contract of the method is that it always returns a String, it is not an option to return nil without making it a breaking change. Also, an empty String concatenation should be harmless to log entries.