Always return :ok even if nothing is logged

[adamu]

Make the return type of telemetry_logging_handler conform to its spec.

Currently, if the if level = level(level, conn) condition fails, the function returns nil, not :ok as specified.

[AndrewDryga]

@adamu thank you! Can you please add a test to make sure this case is covered too? Other than that LGTM.

[coveralls]

coverage: 100.0%. remained the same
when pulling b6a31b8 on adamu:ok
into cad5979 on Nebo15:master.

[adamu]

@AndrewDryga added!