ipaclient: Use omit (None) for password, keytab, no string length checks

Password and keytab do not need to be set explicitely to an empty string
when they are not set. Also there is no need to have string length checks
in the role tasks.

roles/ipaclient/library/ipaclient_join.py

@@ -161,8 +161,7 @@ def main():
     kinit_attempts = module.params.get('kinit_attempts')
     debug = module.params.get('debug')
 
-    if password is not None and password != "" and \
-       keytab is not None and keytab != "":
+    if password is not None and keytab is not None:
         module.fail_json(msg="Password and keytab cannot be used together")
 
     client_domain = hostname[hostname.find(".")+1:]
@@ -174,7 +173,7 @@ def main():
 
     options.ca_cert_file = ca_cert_file
     options.unattended = True
-    options.principal = principal if principal != "" else None
+    options.principal = principal
     options.force = False
     options.password = password
 
@@ -207,7 +206,7 @@ def main():
             env['XMLRPC_TRACE_CURL'] = 'yes'
         if force_join:
             join_args.append("-f")
-        if principal:
+        if principal is not None:
             if principal.find('@') == -1:
                 principal = '%s@%s' % (principal, realm)
             try: