Must exclude from certificate monitoring if SIA's Certificate File or Ca Cert File Path is Empty

[y-myajima]

When starting the metrics server, if either idCfg.CertFile or idCfg.CaCertFile is an empty string, a warning log will be issued due to the specifications of x509-certificate-exporter.

time="2024-08-27T21:40:07+09:00" level=warning msg="failed to parse \"\": stat : invalid argument"

The error is caused by the fs.Stat() function used in x509-certificate-exporter(Relevant section).
If idCfg.CertFile or idCfg.CaCertFile is an empty string, they need to be excluded from certificate monitoring(Relevant section).