[test] fix flaky Vue upload widget browser tests using randomize url param (#5845)

## Summary

Fix browser tests that use filenames in loader nodes. When the files
don't exist, the fallback/error component shows the request URL which
contains random cache-busting param which makes tests fail due to
non-deterministic nature.


[Context](https://github.com/Comfy-Org/ComfyUI_frontend/pull/5831#discussion_r2386582256)

┆Issue is synchronized with this [Notion
page](https://www.notion.so/PR-5845-test-fix-flaky-Vue-upload-widget-browser-tests-using-randomize-url-param-27d6d73d36508107a0d0d52d2764a028)
by [Unito](https://www.unito.io)

---------

Co-authored-by: Claude <noreply@anthropic.com>
Co-authored-by: github-actions <github-actions@github.com>

browser_tests/assets/widgets/all_load_widgets.json

@@ -61,7 +61,7 @@
         "Node name for S&R": "LoadVideo"
       },
       "widgets_values": [
-        "Dying for the right cause, is the most human thing we can do [sOBtQofXPDA].mp4",
+        null,
         "image"
       ]
     },
@@ -96,7 +96,7 @@
         "Node name for S&R": "DevToolsLoadAnimatedImageTest"
       },
       "widgets_values": [
-        "l0isitzgugt41.webp",
+        null,
         "image"
       ]
     },
@@ -131,7 +131,7 @@
         "Node name for S&R": "LoadImage"
       },
       "widgets_values": [
-        "ComfyUI_00084_.png",
+        null,
         "image"
       ]
     },
@@ -161,7 +161,7 @@
         "Node name for S&R": "LoadImageMask"
       },
       "widgets_values": [
-        "01. a lot.mp3",
+        null,
         "alpha",
         "image"
       ]
@@ -197,7 +197,7 @@
         "Node name for S&R": "LoadImageOutput"
       },
       "widgets_values": [
-        "ComfyUI_00509_.png [output]",
+        null,
         false,
         "refresh",
         "image"