add altText checking

Author: mathdroid

src/messages.js

@@ -138,6 +138,10 @@ class Messages {
  }
 
  addButtons ({thumbnailImageUrl, altText, title, text, actions}) {
+ if (!altText) {
+ console.error('altText must not be empty.')
+ return this
+ }
  if (this._payload.length === 5) {
  console.error('Maximum payload length is 5.')
  return this