logging: log_output: Prevent redundant flush with zero-length data

There's such a case captured. When log immediate mode is enabled, each log
message is output per character. However, "log_output_flush()" function is
still called with zero data length at the end of "log_output_process()".
Better to make "buffer_write()" returns immediatley if buffer data
length is zero.

Signed-off-by: Dong Wang <dong.d.wang@intel.com>