Add null check for sign text (PrismarineJS#1553)

Needed in some servers

lib/plugins/blocks.js

@@ -57,7 +57,7 @@ function inject (bot, { version, storageBuilder }) {
         if (data === null || data === '') return ''
 
         const json = JSON.parse(data)
-        if (!('text' in json)) return ''
+        if (json === null || !('text' in json)) return ''
 
         json.text = json.text.replace(/^"|"$/g, '')
         return json