ensure the URL is defined

Former-commit-id: bf6420cec4ec6c0d65a16baa2de72d0f37fb6132

lib/single-file/modules/css-fonts-alt-minifier.js

@@ -216,15 +216,17 @@ this.singlefile.lib.modules.fontsAltMinifier = this.singlefile.lib.modules.fonts
 				} catch (error) {
 					const declarationFontURLs = fontURLs.get(srcDeclaration.data);
 					const fontURL = declarationFontURLs[declarationFontURLs.length - sourceIndex - 1];
-					const fontFace = new FontFace("test-font", "url(" + fontURL + ")");
-					try {
-						await Promise.race(
-							[
-								fontFace.load().then(() => fontFace.loaded).then(() => source.valid = true),
-								new Promise(resolve => setTimeout.call(window, () => { source.valid = true; resolve(); }, FONT_MAX_LOAD_DELAY))
-							]);
-					} catch (error) {
-						// ignored
+					if (fontURL) {
+						const fontFace = new FontFace("test-font", "url(" + fontURL + ")");
+						try {
+							await Promise.race(
+								[
+									fontFace.load().then(() => fontFace.loaded).then(() => source.valid = true),
+									new Promise(resolve => setTimeout.call(window, () => { source.valid = true; resolve(); }, FONT_MAX_LOAD_DELAY))
+								]);
+						} catch (error) {
+							// ignored
+						}
 					}
 				}
 			} else {