Coalesce empty query params to None in /node_startup_errors route

?source= or ?module_name= or ?pack_id= (param present but blank) would have returned {} because the helper treated the empty string as an exact-match filter. Coalesce to None at the route boundary so a present-but-blank query param behaves the same as the param being absent. The helper's own behaviour is unchanged and locked in by a new assertion.

Amp-Thread-ID: https://ampcode.com/threads/T-019e86fd-b68f-74de-8c91-d2662377424a
Co-authored-by: Amp <amp@ampcode.com>

tests-unit/node_startup_errors_test.py

@@ -218,6 +218,11 @@ def test_filter_node_startup_errors_source_filter():
     assert set(grouped["comfy_extras"]) == {"B"}
     # Non-matching source filter returns an empty dict, not an error.
     assert nodes.filter_node_startup_errors(source="nope") == {}
+    # An explicit empty-string filter is treated as a real value (matches
+    # entries whose source is literally ""), NOT silently as "no filter".
+    # The HTTP route layer is responsible for coalescing `?source=` to None
+    # before calling this helper; this assertion locks that contract in.
+    assert nodes.filter_node_startup_errors(source="") == {}
 
 
 def test_filter_node_startup_errors_module_name_filter():