I fixed error with NaN in merged cells

[Magentai]

Hi! 👋

Firstly, thanks for your work on this project! 🙂

Today I used patch-package to patch node-xlsx@0.4.0 for the project I'm working on.

I fixed error with NaN in merged cells:

diff --git a/node_modules/node-xlsx/lib/xlsx/xlsx.js b/node_modules/node-xlsx/lib/xlsx/xlsx.js
index c626943..29fff59 100644
--- a/node_modules/node-xlsx/lib/xlsx/xlsx.js
+++ b/node_modules/node-xlsx/lib/xlsx/xlsx.js
@@ -136,7 +136,7 @@ function xlsx(file, options) {
 					cell = columns[k];
 					f = styles[+getAttr(cell, 's')] || { type: 'General', formatCode: 'General' };
 					t = getAttr(cell, 't') || f.type;
-					val = cell.substring(cell.indexOf('<v>') + 3, cell.indexOf('</v>'));
+					val = cell.indexOf('<v>') !== -1 ? cell.substring(cell.indexOf('<v>') + 3, cell.indexOf('</v>')) : undefined;
 					val = val ? +val : ''; // turn non-zero into number
 					switch (t) {
 						case 's': val = sharedStrings[val]; break;

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