logind: fix property emission on Sessions

logind: fix property emission on Sessions

We would not send the property because we'd call sd_bus_get_current_message()
which would return NULL. If there is no message, we cannot support /self or
/auto, but things are still OK if a path with a session name is given.

Traceback when the issue is triggered:

 #2  we'd call sd_bus_get_current_message() here, which would return NULL, and
     session_object_find() would immediately return 0.
 #3  0x00000000004289b7 in session_object_find (bus=0x9f1110, path=0xa160b0 "/org/freedesktop/login1/session/c2",
     interface=0x9efda0 "org.freedesktop.login1.Session", userdata=0x9852f0, found=0x7ffe3e975fe8, error=0x7ffe3e9760b0)
     at ../src/login/logind-session-dbus.c:620
 #4  0x00007ff74bfdde39 in node_vtable_get_userdata (bus=0x9f1110, path=0xa160b0 "/org/freedesktop/login1/session/c2",
     c=0x9f6d58, userdata=0x7ffe3e976070, error=0x7ffe3e9760b0) at ../src/libsystemd/sd-bus/bus-objects.c:37
 #5  0x00007ff74bfe49af in emit_properties_changed_on_interface (bus=0x9f1110,
     prefix=0xa133a0 "/org/freedesktop/login1/session", path=0xa160b0 "/org/freedesktop/login1/session/c2",
     interface=0x43f9f8 "org.freedesktop.login1.Session", require_fallback=true, found_interface=0x7ffe3e976163,
     names=0x7ffe3e9761b0) at ../src/libsystemd/sd-bus/bus-objects.c:2088
 #6  0x00007ff74bfe56a4 in sd_bus_emit_properties_changed_strv (bus=0x9f1110,
     path=0xa160b0 "/org/freedesktop/login1/session/c2", interface=0x43f9f8 "org.freedesktop.login1.Session",
     names=0x7ffe3e9761b0) at ../src/libsystemd/sd-bus/bus-objects.c:2291
 #7  0x00000000004292ea in session_send_changed (s=0xa16e10, properties=0x43ee27 "Active")
    at ../src/login/logind-session-dbus.c:730
 #8  0x0000000000424cd7 in seat_set_active (s=0x9ee280, session=0xa16e10) at ../src/login/logind-seat.c:249
 #9  0x00000000004251cf in seat_active_vt_changed (s=0x9ee280, vtnr=3) at ../src/login/logind-seat.c:361
 #10 0x000000000042547b in seat_read_active_vt (s=0x9ee280) at ../src/login/logind-seat.c:395
 #11 0x000000000040ab5c in manager_dispatch_console (s=0x9f0320, fd=8, revents=8, userdata=0x9852f0)
     at ../src/login/logind.c:588
 #12 0x00007ff74c042d5f in source_dispatch (s=0x9f0320) at ../src/libsystemd/sd-event/sd-event.c:2828
 #13 0x00007ff74c04469f in sd_event_dispatch (e=0x9ef340) at ../src/libsystemd/sd-event/sd-event.c:3241
 #14 0x00007ff74c044b58 in sd_event_run (e=0x9ef340, timeout=18446744073709551615)
     at ../src/libsystemd/sd-event/sd-event.c:3299
 #15 0x000000000040d7e8 in manager_run (m=0x9852f0) at ../src/login/logind.c:1186
 #16 0x000000000040db58 in run (argc=1, argv=0x7ffe3e976728) at ../src/login/logind.c:1234
 #17 0x000000000040dc30 in main (argc=1, argv=0x7ffe3e976728) at ../src/login/logind.c:1244

Fixes #13437. Bug introduced in 3b92c086a8d5338e2164ffa0ae48b3d03d10cfb5.