Fix NPE by returning empty params.

Re-entrant calls make it possible for inflation to occur after removal.
While http://ag/11334076 prevented the actual inflation from occuring in
this case, it was still possible to hit an NPE by attempting to modify a
notification's inflation parameters that didn't exist. This provides a
temporary workaround by just returning an empty params object.

Bug: 156989744
Test: atest SystemUITests
Change-Id: Ia86a92e0e447e521a6011e7edd32ebdd974a2c6b