fix test case with Optional handling in createSolverByName().

2 jobs for master in 16 minutes and 40 seconds (queued for 8 seconds)
Name Stage Failure
maven38-java11-sonar Build
993168 [ERROR] Re-run Maven using the -X switch to enable full debug logging.
993168 [ERROR]
993168 [ERROR] For more information about the errors and possible solutions, please read the following articles:
993169 [ERROR] [Help 1]
993169 [ERROR]
993169 [ERROR] After correcting the problems, you can resume the build with the command
993170 [ERROR] mvn <args> -rf :org.ow2.sat4j.core
Cleaning up project directory and file based variables
ERROR: Job failed: exit code 1