fix(CoreBot): guard array length access in replaceAttachmentTokens 
fix(CoreBot): guard array length access in replaceAttachmentTokens 

This fixes an error, if attachment is not defined.

    /home/m/www/netwoRHk/node_modules/botkit/lib/CoreBot.js:441
                if (attachments.length) {
                            ^

    TypeError: Cannot read property 'length' of undefined
        at Conversation.replaceAttachmentTokens (/home/m/www/netwoRHk/node_modules/botkit/lib/CoreBot.js:441:28)
        at Conversation.replaceAttachmentTokens (/home/m/www/netwoRHk/node_modules/botkit/lib/CoreBot.js:447:56)
        at Conversation.cloneMessage (/home/m/www/netwoRHk/node_modules/botkit/lib/CoreBot.js:527:45)
        at Conversation.tick (/home/m/www/netwoRHk/node_modules/botkit/lib/CoreBot.js:622:53)
        at Task.tick (/home/m/www/netwoRHk/node_modules/botkit/lib/CoreBot.js:825:36)
        at Object.Botkit.botkit.tick (/home/m/www/netwoRHk/node_modules/botkit/lib/CoreBot.js:1099:29)
        at Timeout._onTimeout (/home/m/www/netwoRHk/node_modules/botkit/lib/CoreBot.js:1049:24)
        at ontimeout (timers.js:365:14)
        at tryOnTimeout (timers.js:237:5)
        at Timer.listOnTimeout (timers.js:207:5)
ouadie-lahdioui/botkit,howdyai/botkit,howdyai/botkit,acrollet/botkit,healthchatnow/hcn,howdyai/botkit,howdyai/botkit