Update rebuild_thumbnails command (#6254)

* Update rebuild_thumbnails command

- Skip if images already exist
- Prevents significant clutter in the logs
- Speeds up the command too

* Refactoring

InvenTree/InvenTree/management/commands/rebuild_thumbnails.py

@@ -4,6 +4,7 @@
 """
 
 import logging
+import os
 
 from django.core.management.base import BaseCommand
 from django.db.utils import OperationalError, ProgrammingError
@@ -26,6 +27,18 @@ class Command(BaseCommand):
 
         img = model.image
 
+        # Check for image paths
+        img_paths = []
+
+        for x in [model.image, model.image.thumbnail, model.image.preview]:
+            if x and x.path:
+                img_paths.append(x.path)
+
+        if len(img_paths) > 0:
+            if all((os.path.exists(path) for path in img_paths)):
+                # All images exist - skip further work
+                return
+
         logger.info("Generating thumbnail image for '%s'", img)
 
         try: