fix: handling of empty discord client event messages

[Ziinc]

GenServer {Logflare.Backends.SourceRegistry, {xxxx, Logflare.Sources.Source.WebhookNotificationServer}} terminating ** (FunctionClauseError) no function clause matching in String.split_at/2 
(elixir 1.19.5) lib/string.ex:700: String.split_at(nil, 1018) 
(logflare 1.34.13) lib/logflare/sources/source/webhook_notification_server/discord_client.ex:102: Logflare.Sources.Source.WebhookNotificationServer.DiscordClient.discord_event_message/1 
(elixir 1.19.5) lib/enum.ex:1688: Enum."-map/2-lists^map/1-1-"/2 
(logflare 1.34.13) lib/logflare/sources/source/webhook_notification_server/discord_client.ex:31: Logflare.Sources.Source.WebhookNotificationServer.DiscordClient.post/4 
(logflare 1.34.13) lib/logflare/sources/source/webhook_notification_server/webhook_notification_server.ex:50: Logflare.Sources.Source.WebhookNotificationServer.handle_info/2 
(stdlib 6.2.2.2) gen_server.erl:2345: :gen_server.try_handle_info/3 
(stdlib 6.2.2.2) gen_server.erl:2433: :gen_server.handle_msg/6 
(stdlib 6.2.2.2) proc_lib.erl:340: :proc_lib.wake_up/3 Last message: :check_rate

[amokan]

Looks good and so sorry for the delay on this @Ziinc

Minor comment below about a guard we have for the binary not empty string check.