[firestore][js] additional Blob class input arg validations

lib/modules/firestore/Blob.js

@@ -14,6 +14,12 @@ export default class Blob {
    * @param base64 string
    */
   static fromBase64String(base64: string): Blob {
+    if (typeof base64 !== 'string' || base64.length < 1) {
+      throw new Error(
+        'firestore.Blob.fromBase64String expects a string of at least 1 character in length'
+      );
+    }
+
     return new Blob(Base64.atob(base64));
   }