fix(syslog): avoid nil logger:log(log_message) when init fails

[Arjen10]

Description

return false and the init error message to avoid calling logger:log on a nil logger

Checklist

• I have explained the need for this PR and the problem it solves
• I have explained the changes or the new features added to this PR
• I have added tests corresponding to this change
• I have updated the documentation to reflect this change
• I have verified that this change is backward compatible (If not, please discuss on the APISIX mailing list first)