IpInterfaceScan shouldDetect() method should check for empty string in addition to null string

I'm writing a detector for protocol XMP.

My XmpDetector was returning empty string for ipMatch method and IpInterfaceScan shouldDetect() method was only checking for null string. Null string and empty string are semantically same but code was not catching.

I added check for empty string to shouldDetect() and am testing now. I will submit enhancement with my XmpDetector via my pull request.

Thanks,

Bobby