make creds checking a requirement for sharenum

[atucom]

https://github.com/atucom/dotfiles/blob/master/bin/atu-shareEnum.rb

when you dont specify creds:
/test/dotfiles/bin/atu-shareEnum.rb:31:in parseSMBCreds': undefined methodsplit' for nil:NilClass (NoMethodError)
from /test/dotfiles/bin/atu-shareEnum.rb:66:in `

Also, allow for null sessions.