support for None and empty strings
support for None and empty strings

username and passwords can now be `None` or `''`
added `else` statements to LDAP, NTLM, PKI, and OAUTH as a `NoneType`
error was being thrown. Since usernames/passwords keys/certs
client/secret IDs are required for each of these, the securityhandler
should not proceed

BrunoCaimar/ArcREST,adegwerth/ArcREST,Esri/ArcREST,DShokes/ArcREST,pLeBlanc93/ArcREST