XLSX SST treat <si></si> as empty (fixes #2083)

Author: Martin

bits/42_sstxml.js

@@ -169,7 +169,7 @@ var sirphregex = /<(?:\w+:)?rPh.*?>([\s\S]*?)<\/(?:\w+:)?rPh>/g;
 function parse_si(x, opts) {
 	var html = opts ? opts.cellHTML : true;
 	var z = {};
-	if(!x) return null;
+	if(!x) return { t: "" };
 	//var y;
 	/* 18.4.12 t ST_Xstring (Plaintext String) */
 	// TODO: is whitespace actually valid here?