Make the Ruby code error-safe

Author: Takahiro-Yoko

modules/exploits/linux/http/appsmith_rce_cve_2024_55964.rb

@@ -97,7 +97,7 @@ def exploit
     })
     fail_with(Failure::Unknown, 'Failed to access workspaces.') unless res&.code == 200
 
-    workspace_id = res.get_json_document['data'][0]['id']
+    workspace_id = res.get_json_document&.dig('data')&.first&.dig('id')
 
     res = send_request_cgi({
       'method' => 'GET',