fix(color): don't throw on empty computed color (AXE-3861)

lib/commons/color/color.js

@@ -130,6 +130,16 @@ export default class Color {
    * @instance
    */
   parseString(colorString) {
+    // [a11y-rule-color-contrast]: a detached/inert node's computed *-color resolves
+    // to '' (AXE-3861); colorjs throws "Empty color reference" and that abort takes
+    // down the whole in-page Type-B collection. Treat blank as transparent, not fatal.
+    if (typeof colorString !== 'string' || colorString.trim() === '') {
+      this.red = 0;
+      this.green = 0;
+      this.blue = 0;
+      this.alpha = 0;
+      return this;
+    }
     // Colorjs <v0.5.0 does not support rad or turn angle values
     // @see https://github.com/LeaVerou/color.js/issues/311
     colorString = colorString.replace(hslRegex, (match, angle, unit) => {

test/commons/color/color.js

@@ -417,6 +417,30 @@ describe('color.Color', () => {
       assert.equal(c.blue, 144);
       assert.equal(c.alpha, 0.5);
     });
+
+    // [a11y-rule-color-contrast]: AXE-3861 — a detached/inert node's computed
+    // *-color resolves to ''; colorjs throws "Empty color reference" and the abort
+    // takes down the whole in-page Type-B collection. Guard: blank -> transparent.
+    it('does not throw on an empty string (returns transparent)', () => {
+      const c = new Color(255, 255, 255, 1);
+      assert.doesNotThrow(() => c.parseString(''));
+      assert.equal(c.red, 0);
+      assert.equal(c.green, 0);
+      assert.equal(c.blue, 0);
+      assert.equal(c.alpha, 0);
+    });
+
+    it('does not throw on a whitespace-only string', () => {
+      const c = new Color();
+      assert.doesNotThrow(() => c.parseString('   '));
+      assert.equal(c.alpha, 0);
+    });
+
+    it('does not throw on a non-string value', () => {
+      const c = new Color();
+      assert.doesNotThrow(() => c.parseString(null));
+      assert.equal(c.alpha, 0);
+    });
   });
 
   describe('toHexString', () => {