Fix: query TYPE_WIFI usage with empty string

Starting with API level 31, the subscriberId is applicable
for the wifi network. Considering applications may use
null or an empty string as subscriberId (for instance, cts),
frameworks create MATCH_WIFI_WILDCARD NetworkTemplate when querying
wifi network with null or an empty string which is the behavior before
API level 31.

Bug: 188915450
Test: atest -c NetworkStatsManagerTest

Change-Id: I084b69903f8ba7a6225b312560752e8508938714