Avoid a bunch of log spam in SmartReplyConstantsTest.

DeviceConfig logs errors when trying to read e.g. an integer and the
value provided isn't an integer. However, DeviceConfig avoids logging
the error if the value is null. So with this CL we use null as the reset
value for SmartReplyConstantsTest.

Bug: 129538548
Test: atest SmartReplyConstantsTest, and check logcat for spam.
Change-Id: I4c714b7e808163c20fccc5cdca734a255c698ede