Replace non valid values with NaN instead of "" in csv export (#660)

* Replace non valid values with 0 instead of "" in csv export

* Changes according to PR comments

* Removed redundant double negation

* Removed unnecessary use of conditional expression for default assignment.

* Update js/webworkers/csv-export-worker.js

Co-authored-by: Petr Ledvina <[email protected]>

---------

Co-authored-by: Petr Ledvina <[email protected]>

Author: pichim

js/webworkers/csv-export-worker.js

@@ -25,14 +25,29 @@ onmessage = function(event) {
             .join(opts.columnDelimiter);
     }
 
+    /**
+     * Converts `null` entries in columns and other empty non-numeric values to NaN value string.
+     * 
+     * @param {array} columns 
+     * @returns {string}
+     */
+    function joinColumnValues(columns) {
+        return _(columns)
+            .map(value => 
+                (_.isNumber(value) || _.value)
+                ? value
+                : "NaN")
+            .join(opts.columnDelimiter);
+    }
+
     let opts = event.data.opts,
         stringDelim = opts.quoteStrings
             ? opts.stringDelimiter
             : "",
         mainFields = _([joinColumns(event.data.fieldNames)])
             .concat(_(event.data.frames)
                 .flatten()
-                .map(row => joinColumns(row))
+                .map(row => joinColumnValues(row))
                 .value())
             .join("\n"),
         headers = _(event.data.sysConfig)