Fix #136252: Extra chevron on Particle context data breadcrumbs

Dont increment the path count (and dont create a corresponding pointer)
in `ButsContextPath` when there is no current particle system.
This alone would do the trick for particles, but I have also changed
`buttons_panel_context_draw` slightly (so that it checks if the pointer
is valid -- earlying out otherwise -- **before** drawing the triangle
icon) in case it might happen elsewhere too.

Pull Request: https://projects.blender.org/blender/blender/pulls/136256

source/blender/editors/space_buttons/buttons_context.cc

@@ -437,9 +437,10 @@ static bool buttons_context_path_particle(ButsContextPath *path)
 
     if (ob && ob->type == OB_MESH) {
       ParticleSystem *psys = psys_get_current(ob);
-
-      path->ptr[path->len] = RNA_pointer_create_discrete(&ob->id, &RNA_ParticleSystem, psys);
-      path->len++;
+      if (psys != nullptr) {
+        path->ptr[path->len] = RNA_pointer_create_discrete(&ob->id, &RNA_ParticleSystem, psys);
+        path->len++;
+      }
       return true;
     }
   }
@@ -1226,15 +1227,15 @@ static void buttons_panel_context_draw(const bContext *C, Panel *panel)
       continue;
     }
 
+    if (ptr->data == nullptr) {
+      continue;
+    }
+
     /* Add > triangle. */
     if (!first) {
       uiItemL(row, "", ICON_RIGHTARROW);
     }
 
-    if (ptr->data == nullptr) {
-      continue;
-    }
-
     /* Add icon and name. */
     int icon = RNA_struct_ui_icon(ptr->type);
     char namebuf[128];