logging: Minor improvement in instance logging

When using instance logging (e.g. LOG_INST_INF) first argument is
a pointer to the logging object associated with an instance. It is
not used if logging is disabled and that can generate 'unused
variable' warning when logging is disabled.

Previously logging object was not created when logging was disabled
That was done to save memory but because of that object could not be
referenced when logging was disabled. Better approach is to create
empty array instead of a logging object. It does not increase memory
usage but can be referenced and potential compilation warning is
suppressed.

Signed-off-by: Krzysztof Chruściński <krzysztof.chruscinski@nordicsemi.no>