fix: fillNa throws an error when value is empty string

Dataframe.fillNa("") throws an error. 
There are few use cases when we want to fill NaN with an empty string.
This feature is supported by pandas.

Author: ankitskvmdam

src/danfojs-base/core/frame.ts

@@ -2102,7 +2102,7 @@ export default class DataFrame extends NDframe implements DataFrameInterface {
     ): DataFrame | void {
         let { columns, inplace } = { inplace: false, ...options }
 
-        if (!values && typeof values !== "boolean" && typeof values !== "number") {
+        if (!values && typeof values !== "boolean" && typeof values !== "number" && typeof values !== "string") {
             throw Error('ParamError: value must be specified');
         }