libtracker-sparql: Allow NULL store in tracker_sparql_connection_new_async()

Make it on par with tracker_sparql_connection_new(), since a NULL store means
"use in-memory database".
4 jobs for wip/carlosg/portal in 7 minutes and 29 seconds (queued for 1 second)
Status Name Job ID Coverage
  Test
passed test-alpine-edge #814715

00:03:00

passed test-fedora-latest #814716

00:03:24

passed test-ubuntu-rolling #814717

00:00:54

 
  Analysis
passed coverage-analysis #814718

00:04:05

72.0%