fix small ts issue

Author: theoephraim

src/lib/GoogleSpreadsheet.ts

@@ -157,7 +157,7 @@ export class GoogleSpreadsheet {
         : authConfig.headers;
 
       Object.entries(headers).forEach(([key, val]) => {
-        config.headers.set(key, val);
+        config.headers.set(key, String(val));
       });
     }
 

src/lib/GoogleSpreadsheetCell.ts

@@ -146,7 +146,7 @@ export class GoogleSpreadsheetCell {
 
   /** a note attached to the cell */
   get note(): string {
-    return this._draftData.note !== undefined ? this._draftData.note : this._rawData?.note;
+    return this._draftData.note !== undefined ? this._draftData.note : this._rawData?.note || '';
   }
   set note(newVal: string | null | undefined | false) {
     if (newVal === null || newVal === undefined || newVal === false) newVal = '';