Line numbers: Fixed TypeError when lineNumberWrapper is null (#2337)

If a code block doesn't contain code, line numbers will not be added but the resize event still expected them to be there resulting in a TypeError.

Author: lynette-li

plugins/line-numbers/prism-line-numbers.js

@@ -27,6 +27,9 @@
 		if (whiteSpace === 'pre-wrap' || whiteSpace === 'pre-line') {
 			var codeElement = element.querySelector('code');
 			var lineNumbersWrapper = element.querySelector('.line-numbers-rows');
+			if (!codeElement || !lineNumbersWrapper) {
+				return;
+			}
 			var lineNumberSizer = element.querySelector('.line-numbers-sizer');
 			var codeLines = codeElement.textContent.split(NEW_LINE_EXP);