improved removal of unused fonts (fix #313)

Former-commit-id: 1e43cbd3f48588e68f97cd60c3d695bc1c930eb5

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

@@ -72,7 +72,9 @@ this.singlefile.lib.modules.fontsMinifier = this.singlefile.lib.modules.fontsMin
 				fontsInfo.used.forEach(fontNames => fontNames.forEach(familyName => {
 					if (fontsInfo.declared.find(fontInfo => fontInfo.fontFamily == familyName)) {
 						const optionalData = options.usedFonts && options.usedFonts.filter(fontInfo => fontInfo[0] == familyName);
-						filteredUsedFonts.set(familyName, optionalData);
+						if (optionalData && optionalData.length) {
+							filteredUsedFonts.set(familyName, optionalData);
+						}
 					}
 				}));
 				unusedFonts = fontsInfo.declared.filter(fontInfo => !filteredUsedFonts.has(fontInfo.fontFamily));