Fix "null" toast when FillCallback.onFailure(null) called.

Root cause
The error message is set by AutofillService, AutofillService calls
FillCallback.onFailure(null) back to the platform. Because a081250c
uses String.valueOf(message), this method will make a null object
become to a "null" string. This may cause the Autofill session to
think we should show message because of a non-null message. And if
the AutofillService target SDK is lower than 29, we don't ignore show
message, then users will see a "null" toast.

Solution
Instead of passing the message parameter to String.valueOf() directly
, we check the message first, only passing to String.valueOf() if the
message is not null. And we pass an empty string if the message
parameter is null.

Bug: 159888723
Test: Use a test AutofillService which target SDK is lower than 29,
make sure no null toast is shown
Test: atest CtsAutoFillServiceTestCases:LoginActivityTest#\
testAutofillAgainAfterOnFailure

Change-Id: Id7ff581b3a7f35cf97b4f3405a40bfa1a420823e