remove empty media rules

lib/single-file/css-medias-minifier.js

@@ -41,9 +41,11 @@ this.mediasMinifier = this.mediasMinifier || (() => {
 				if (cssRule.type == CSSRule.MEDIA_RULE) {
 					stats.processed++;
 					if (matchesMediaType(cssRule.media.mediaText, "screen")) {
-						sheetContent += "@media " + Array.from(cssRule.media).join(",") + "{";
-						sheetContent += processRules(doc, cssRule.cssRules, cssRule.media.mediaText, stats);
-						sheetContent += "}";
+						if (cssRule.cssRules.length) {
+							sheetContent += "@media " + Array.from(cssRule.media).join(",") + "{";
+							sheetContent += processRules(doc, cssRule.cssRules, cssRule.media.mediaText, stats);
+							sheetContent += "}";
+						}
 					} else {
 						stats.discarded++;
 					}