player/command: don't synthesize property names with a trailing /

This is not only cosmetic, but also avoids dummy get key action with
empty key. While this was allowed previously, I'm bit concerned about
Hyrum's law. But let's be strict about it.

This commit fixes current-tracks property.

Fixes: 95019fc

player/command.c

@@ -2290,7 +2290,7 @@ static int mp_property_current_tracks(void *ctx, struct m_property *prop,
     }
     assert(index >= 0);
 
-    char *name = mp_tprintf(80, "track-list/%d/%s", index, rem);
+    char *name = mp_tprintf(80, "track-list/%d%s%s", index, *rem ? "/" : "", rem);
     return mp_property_do(name, ka->action, ka->arg, ctx);
 }