Fix mimikatz error when password is nil

In some cases the password value that comes out of mimikatz results
is `nil`, instead of an empty string. This fixes this so that if
the string is `nil` is falls back to an empty string, resulting in
the call to `gsub` working instead of failing.

Author: OJ

lib/rex/post/meterpreter/ui/console/command_dispatcher/mimikatz.rb

@@ -106,7 +106,7 @@ def mimikatz_request(provider, method)
     )
 
     accounts.each do |acc|
-      table << [acc[:authid], acc[:package], acc[:domain], acc[:user],  acc[:password].gsub("\n","")]
+      table << [acc[:authid], acc[:package], acc[:domain], acc[:user], (acc[:password] || "").gsub("\n","")]
     end
 
     print_line table.to_s