core: use UNIT_TRIGGER at more places

Just some minor simplification.

src/core/load-fragment.c

@@ -2015,7 +2015,7 @@ int config_parse_trigger_unit(
         assert(rvalue);
         assert(data);
 
-        if (!hashmap_isempty(u->dependencies[UNIT_TRIGGERS])) {
+        if (UNIT_TRIGGER(u)) {
                 log_syntax(unit, LOG_WARNING, filename, line, 0, "Multiple units to trigger specified, ignoring: %s", rvalue);
                 return 0;
         }

src/core/path.c

@@ -316,7 +316,7 @@ static int path_add_trigger_dependencies(Path *p) {
 
         assert(p);
 
-        if (!hashmap_isempty(UNIT(p)->dependencies[UNIT_TRIGGERS]))
+        if (UNIT_TRIGGER(UNIT(p)))
                 return 0;
 
         r = unit_load_related_unit(UNIT(p), ".service", &x);

src/core/timer.c

@@ -124,7 +124,7 @@ static int timer_add_trigger_dependencies(Timer *t) {
 
         assert(t);
 
-        if (!hashmap_isempty(UNIT(t)->dependencies[UNIT_TRIGGERS]))
+        if (UNIT_TRIGGER(UNIT(t)))
                 return 0;
 
         r = unit_load_related_unit(UNIT(t), ".service", &x);