tests: return 0 instead of None for unknown file in test component class support...

tests: return 0 instead of None for unknown file in test component class support-info query

I noticed some spurious warnings while running
TestAutoDiscoverSourceComponentSpecsGrouping.test_grouping (shortened
for brevity):

    unexpected result type: component-class-name=source.test.TestSourceExt, input=/home/smarchi/src/babeltrace/tests/data/auto-source-discovery/grouping/bt_plugin_test.py, input-type=file, expected-types=REAL,MAP,SIGNED_INTEGER, actual-type=NULL

Indeed, the _query method of TestSourceExt fails to return a value (so
it returns None) if it doesn't know the file.  This is not on purpose in
this test, so fix it.

Change-Id: I303c78dae4a518c62573263aa7a3afafd0c61ed7
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/1906
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>
Tested-by: jenkins <jenkins@lttng.org>