fix: CSS coverage should work with empty stylesheets (#8570)

Closes #8535

src/common/Coverage.ts

@@ -407,9 +407,15 @@ export class CSSCoverage {
     const coverage: CoverageEntry[] = [];
     for (const styleSheetId of this.#stylesheetURLs.keys()) {
       const url = this.#stylesheetURLs.get(styleSheetId);
-      assert(url);
+      assert(
+        typeof url !== 'undefined',
+        `Stylesheet URL is undefined (styleSheetId=${styleSheetId})`
+      );
       const text = this.#stylesheetSources.get(styleSheetId);
-      assert(text);
+      assert(
+        typeof text !== 'undefined',
+        `Stylesheet text is undefined (styleSheetId=${styleSheetId})`
+      );
       const ranges = convertToDisjointRanges(
         styleSheetIdToCoverage.get(styleSheetId) || []
       );

test/assets/csscoverage/empty.html

@@ -0,0 +1,3 @@
+<style></style>
+<div>empty style tag</div>
+

test/src/coverage.spec.ts

@@ -270,6 +270,15 @@ describe('Coverage specs', function () {
         JSON.stringify(coverage, null, 2).replace(/:\d{4}\//g, ':<PORT>/')
       ).toBeGolden('csscoverage-involved.txt');
     });
+    it('should work with empty stylesheets', async () => {
+      const {page, server} = getTestState();
+
+      await page.coverage.startCSSCoverage();
+      await page.goto(server.PREFIX + '/csscoverage/empty.html');
+      const coverage = await page.coverage.stopCSSCoverage();
+      expect(coverage.length).toEqual(1);
+      expect(coverage[0]!.text).toEqual('');
+    });
     it('should ignore injected stylesheets', async () => {
       const {page} = getTestState();