This commit fixes an issue with the use of None in default values
for the config get() calls in __init__() of AsusWrtDeviceScanner.
These values are cast as strings and when a NoneType is cast it
returns the string "None" this broke the check for the existence
of these fields. This commit fixes the issue by changing the default
value to be an empty string '' which will conform with the behavior
expected by the ssh login code.
Closes#2343
aa3d0e1047