Handle null response in onError handler

[glenn-allen]

Hi guys,

Just discovered an error that might be occurring since we upgraded to Node v16.13.2 (from 12), though it's potentially been happening for previous versions and we've only noticed it now because it now crashes our servers - due to the unhandled exception change in Node.

Anyway, it seems the onError handler isn't safe if the response is null. https://github.com/filepreviews/filepreviews-node/blob/3c6c5f3ed14993216377c2f41f2533379ec327e2/lib/index.js#L89-L92

See the stack trace below:

/home/ubuntu/www/node_modules/filepreviews/lib/index.js:92
    this.log('API request error: ' + response.statusCode);
                                              ^

TypeError: Cannot read properties of undefined (reading 'statusCode')
    at FilePreviews.<anonymous> (/home/ubuntu/www/node_modules/filepreviews/lib/index.js:92:47)
    at Request._callback (/home/ubuntu/www/node_modules/filepreviews/lib/index.js:135:9)
    at self.callback (/home/ubuntu/www/node_modules/request/request.js:185:22)
    at Request.emit (node:events:390:28)
    at Request.emit (node:domain:475:12)
    at Request.onRequestError (/home/ubuntu/www/node_modules/request/request.js:877:8)
    at ClientRequest.emit (node:events:390:28)
    at ClientRequest.emit (node:domain:475:12)
    at TLSSocket.socketErrorListener (node:_http_client:447:9)
    at TLSSocket.emit (node:events:390:28)
    at TLSSocket.emit (node:domain:475:12)
    at emitErrorNT (node:internal/streams/destroy:157:8)
    at emitErrorCloseNT (node:internal/streams/destroy:122:3)
    at processTicksAndRejections (node:internal/process/task_queues:83:21)

Thanks!