Fix opt address local crash when ipaddr is nil

[adfoster-r7]

Fixes the following crash with OptAddressLocal:

  1) Msf::OptAddressLocal behaves like an option with valid values should be valid and normalize appropriately: utun2
     Failure/Error: IPAddr.new(addr)
     
     IPAddr::AddressFamilyError:
       address family must be specified
 

Caused by darwin AF_LINK having an empty string for its addr

Verification

Ensure CI passes

[cgranleese-r7]

Release Notes

This PR fixes a crash with OptAddressLocal that was caused by darwin AF_LINK having an empty string for its addr.