Fix exception when image upload fails

[HeyBanditoz]

Sep 07 16:45:30 banditoz.io java[8004]: 16:45:30:272 [JDA MainWS-ReadThread] ERROR i.b.g.events.DiscordMessageEvent - Exception on building attachments!
Sep 07 16:45:30 banditoz.io java[8004]: java.lang.NullPointerException: null
Sep 07 16:45:30 banditoz.io java[8004]:         at io.banditoz.gmecord.api.Payload.getPictureUrl(Payload.java:20)
Sep 07 16:45:30 banditoz.io java[8004]:         at io.banditoz.gmecord.util.SerializerDeserializer.deserializeImageGivenString(SerializerDeserializer.java:33)
Sep 07 16:45:30 banditoz.io java[8004]:         at io.banditoz.gmecord.GroupmeImageUploader.uploadImage(GroupmeImageUploader.java:46)
Sep 07 16:45:30 banditoz.io java[8004]:         at io.banditoz.gmecord.util.BuildAttachments.buildImageAttachments(BuildAttachments.java:23)
Sep 07 16:45:30 banditoz.io java[8004]:         at io.banditoz.gmecord.events.DiscordMessageEvent.onMessageReceived(DiscordMessageEvent.java:30)
Sep 07 16:45:30 banditoz.io java[8004]:         at net.dv8tion.jda.api.hooks.ListenerAdapter.onEvent(ListenerAdapter.java:375)
Sep 07 16:45:30 banditoz.io java[8004]:         at net.dv8tion.jda.api.hooks.InterfacedEventManager.handle(InterfacedEventManager.java:96)
Sep 07 16:45:30 banditoz.io java[8004]:         at net.dv8tion.jda.internal.hooks.EventManagerProxy.handle(EventManagerProxy.java:64)
Sep 07 16:45:30 banditoz.io java[8004]:         at net.dv8tion.jda.internal.JDAImpl.handleEvent(JDAImpl.java:148)
Sep 07 16:45:30 banditoz.io java[8004]:         at net.dv8tion.jda.internal.handle.MessageCreateHandler.handleInternally(MessageCreateHandler.java:122)
Sep 07 16:45:30 banditoz.io java[8004]:         at net.dv8tion.jda.internal.handle.SocketHandler.handle(SocketHandler.java:37)
Sep 07 16:45:30 banditoz.io java[8004]:         at net.dv8tion.jda.internal.requests.WebSocketClient.onDispatch(WebSocketClient.java:838)
Sep 07 16:45:30 banditoz.io java[8004]:         at net.dv8tion.jda.internal.requests.WebSocketClient.onEvent(WebSocketClient.java:737)
Sep 07 16:45:30 banditoz.io java[8004]:         at net.dv8tion.jda.internal.requests.WebSocketClient.handleEvent(WebSocketClient.java:716)
Sep 07 16:45:30 banditoz.io java[8004]:         at net.dv8tion.jda.internal.requests.WebSocketClient.onBinaryMessage(WebSocketClient.java:876)
Sep 07 16:45:30 banditoz.io java[8004]:         at com.neovisionaries.ws.client.ListenerManager.callOnBinaryMessage(ListenerManager.java:368)
Sep 07 16:45:30 banditoz.io java[8004]:         at com.neovisionaries.ws.client.ReadingThread.callOnBinaryMessage(ReadingThread.java:270)
Sep 07 16:45:30 banditoz.io java[8004]:         at com.neovisionaries.ws.client.ReadingThread.handleBinaryFrame(ReadingThread.java:990)
Sep 07 16:45:30 banditoz.io java[8004]:         at com.neovisionaries.ws.client.ReadingThread.handleFrame(ReadingThread.java:749)
Sep 07 16:45:30 banditoz.io java[8004]:         at com.neovisionaries.ws.client.ReadingThread.main(ReadingThread.java:108)
Sep 07 16:45:30 banditoz.io java[8004]:         at com.neovisionaries.ws.client.ReadingThread.runMain(ReadingThread.java:64)
Sep 07 16:45:30 banditoz.io java[8004]:         at com.neovisionaries.ws.client.WebSocketThread.run(WebSocketThread.java:45)