fix WifiUtils:isNetworkLockedDown() null pointer issue

In the WifiUtils class, the isNetworkLockedDown() method does not
determine whether the context is empty, which will cause a null
pointer exception. This change adds a condition for judging the
input context as empty.

Bug:261812909
Test:make RunSettingsRoboTests
Change-Id: Ie0f17179c25a7e9d4b2f652a5667eef3cc577c7b
Signed-off-by: xuelin <xuelin@xiaomi.corp-partner.google.com>