[moblab] Change the default wifi password to ''

Currnetly if there is no wifi password configured we are passing
the string None as the password change to empty string.

TEST=built fizz moblab
BUG=chromium:814467

Change-Id: I6d3b99ae636cb7c183283b45bd6bc4122508955b
Reviewed-on: https://chromium-review.googlesource.com/932543
Commit-Ready: Keith Haddow <haddowk@chromium.org>
Tested-by: Keith Haddow <haddowk@chromium.org>
Reviewed-by: Keith Haddow <haddowk@chromium.org>
Reviewed-by: Matt Mallett <mattmallett@chromium.org>
(cherry picked from commit b5b377277110180ea986e12c0b2e3b7c64e1102c)
Reviewed-on: https://chromium-review.googlesource.com/933708
Commit-Queue: Keith Haddow <haddowk@chromium.org>
Trybot-Ready: Keith Haddow <haddowk@chromium.org>
diff --git a/frontend/afe/moblab_rpc_interface.py b/frontend/afe/moblab_rpc_interface.py
index 14629b4..139890f 100644
--- a/frontend/afe/moblab_rpc_interface.py
+++ b/frontend/afe/moblab_rpc_interface.py
@@ -939,7 +939,7 @@
 
     ap_name =_CONFIG.get_config_value('MOBLAB', _WIFI_AP_NAME, default=None)
     test_args['ssid'] = ap_name
-    ap_pass =_CONFIG.get_config_value('MOBLAB', _WIFI_AP_PASS, default=None)
+    ap_pass =_CONFIG.get_config_value('MOBLAB', _WIFI_AP_PASS, default='')
     test_args['wifipass'] = ap_pass
 
     afe = frontend.AFE(user='moblab')