Allow multi-files variants to have no parameters in canvas WPT generator

It was already possible to have variants without parameters, but it was
only working for single-file variant layout. This CL fixed it for
multi-files, allowing tests like:

- name: 2d.example-empty-params
  code: ctx.globalCompositeOperation = {{ variant_name }};
  variants:
  - source-over:
    source-in:
    source-out:

Change-Id: Ibf4584cb693f8e2aa2a1dc432baf303c98812cbe
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6227183
Reviewed-by: Aaron Krajeski <aaronhk@chromium.org>
Commit-Queue: Jean-Philippe Gravel <jpgravel@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1424216}

html/canvas/tools/gentest.py

@@ -1349,7 +1349,7 @@ def _get_variant_grids(
         variants = dimension.variants
         if dimension.layout == _VariantLayout.MULTI_FILES:
             grids = [
-                grid.merge_params(name, params)
+                grid.merge_params(name, params or {})
                 for name, params in variants.items() for grid in grids
             ]
         else: