rcutorture: Don't keep empty console.log.diags files

Currently, an error-free run produces an empty console.log.diags file.
This can be annoying when using "vi */console.log.diags" to see a full
summary of the errors.  This commit therefore removes any empty files
during the analysis process.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>