Fix invalid matrix-postgres.service when matrix_postgres_process_extra_arguments is empty

This only seems to be affecting some people badly enough to cause
matrix-postgres not to start. Certain systemd versions probably handle
it better or something.

Fixes https://github.com/spantaleev/matrix-docker-ansible-deploy/issues/889
(hopefully)

roles/matrix-postgres/templates/systemd/matrix-postgres.service.j2

@@ -29,10 +29,7 @@ ExecStart={{ matrix_host_command_docker }} run --rm --name matrix-postgres \
 			{{ arg }} \
 			{% endfor %}
 			{{ matrix_postgres_docker_image_to_use }} \
-			postgres \
-			{% for arg in matrix_postgres_process_extra_arguments %}
-			{{ arg }} \
-			{% endfor %}
+			postgres {{ matrix_postgres_process_extra_arguments|join(' ') }}
 
 ExecStop=-{{ matrix_host_command_sh }} -c '{{ matrix_host_command_docker }} kill matrix-postgres 2>/dev/null'
 ExecStop=-{{ matrix_host_command_sh }} -c '{{ matrix_host_command_docker }} rm matrix-postgres 2>/dev/null'