support not setting `default` in variant when using `PARAGON_THEME_URLS`

[brian-smith-tcril]

If we add the following test

    it('returns expected object when only brand override URLs are present, fallback to PARAGON_THEME', () => {
      const config = {
        PARAGON_THEME_URLS: {
          core: {
            urls: {
              brandOverride: 'https://www.example.com/example-brand-core.css',
            },
          },
          defaults: {
            light: 'light',
          },
          variants: {
            light: {
              urls: {
                brandOverride: 'https://www.example.com/example-brand-light.css'
              },
            }
          },
        },
      };
      mergeConfig(config);
      const { result } = renderHook(() => useParagonThemeUrls());
      expect(result.current).toEqual(
        expect.objectContaining({
          core: {
            urls: {
              default: '//localhost:8080/local-core.min.css',
              brandOverride: 'https://www.example.com/example-brand-core.css',
            },
          },
          defaults: {
            light: 'light',
          },
          variants: {
            light: {
              urls: {
                default: '//localhost:8080/local-light.min.css',
                brandOverride: 'https://www.example.com/example-brand-light.css',
              },
            },
          },
        }),
      );
    });

to useParagonThemeUrls.test.js it will fail because actual result will have an undefined for default in light

@@ -10,10 +10,10 @@
    },
    "variants": Object {
      "light": Object {
        "urls": Object {
          "brandOverride": "https://www.example.com/example-brand-light.css",
-         "default": "//localhost:8080/local-light.min.css",
+         "default": undefined,
        },
      },
    },
  }

Considering core handles not having a URL for default gracefully, it would be nice to have the variants handle that gracefully too.

[adamstankiewicz]

I agree with this suggestion, especially if we already happen to support the default fallback for the core styles.

[dcoa]

I think it is a use case that was not taken in account before, and I agree that should be address, I think the main reason why we don't considered before was the assumption that if you want to use only one url the user will use the syntax (

frontend-platform/src/react/hooks/paragon/useParagonThemeUrls.js

Line 103 in 9f92281

// If we don't have the core default or any theme variants, use the PARAGON_THEME

):

const config = {
        PARAGON_THEME_URLS: {
          core: {
            url: 'core.css',
          },
          defaults: {
            light: 'light',
          },
          variants: {
            light: {
              url: 'light.css',
            },
          },
        },
      };

I will have a look

[brian-smith-tcril]

I think it is a use case that was not taken in account before, and I agree that should be address, I think the main reason why we don't considered before was the assumption that if you want to use only one url the user will use the syntax [...]

The issue with only using 1 url instead of a urls object in this scenario is that a single url is treated as default, not as brandOverride

frontend-platform/src/react/hooks/paragon/useParagonThemeUrls.js

Lines 78 to 85 in 9f92281

	if (url) {
	themeVariantMetadata.urls = {
	default: handleVersionSubstitution({
	url,
	wildcardKeyword: '$paragonVersion',
	localVersion: localParagonVersion,
	}),
	};

I do think treating the 1 url as default makes sense as the standard behavior, I'd just like to support the "just override a couple things" use case nicely.

[brian-smith-tcril]

#810