BUG/MEDIUM: dns: DNS resolution doesn't start

Patch f046f1156149d3d8563cc45d7608f2c42ef5b596 introduced a regression:
DNS resolution doesn't start anymore, while it was supposed to make it
start with first health check.

Current patch fix this issue by triggering a new DNS resolution if the
last_resolution time is not set.
diff --git a/src/checks.c b/src/checks.c
index e386bee..3fb166b 100644
--- a/src/checks.c
+++ b/src/checks.c
@@ -2155,7 +2155,7 @@
 			 * if there has not been any name resolution for a longer period than
 			 * hold.valid, let's trigger a new one.
 			 */
-			if (tick_is_expired(tick_add(resolution->last_resolution, resolution->resolvers->hold.valid), now_ms)) {
+			if (!resolution->last_resolution || tick_is_expired(tick_add(resolution->last_resolution, resolution->resolvers->hold.valid), now_ms)) {
 				trigger_resolution(s);
 			}
 		}