_pruneEvents(): TypeError: Cannot read property 'length' of null

## Expected Behavior

Events have to be emitted.

## Current Behavior

Some events are not sent to the Amplitude because of an error:
`2020-12-08T23:15:37: TypeError: Cannot read property 'length' of null
2020-12-08T23:15:37: at /var/www/socialcomm/source/server/node_modules/@amplitude/node/src/retryHandler.ts:91:18
2020-12-08T23:15:37: at Array.forEach ()
2020-12-08T23:15:37: at RetryHandler._pruneEvents (/var/www/socialcomm/source/server/node_modules/@amplitude/node/src/retryHandler.ts:89:12)
2020-12-08T23:15:37: at RetryHandler. (/var/www/socialcomm/source/server/node_modules/@amplitude/node/src/retryHandler.ts:51:31)
2020-12-08T23:15:37: at step (/var/www/socialcomm/source/server/node_modules/tslib/tslib.js:141:27)
2020-12-08T23:15:37: at Object.next (/var/www/socialcomm/source/server/node_modules/tslib/tslib.js:122:57)
2020-12-08T23:15:37: at /var/www/socialcomm/source/server/node_modules/tslib/tslib.js:115:75
2020-12-08T23:15:37: at new Promise ()
2020-12-08T23:15:37: at Object.__awaiter (/var/www/socialcomm/source/server/node_modules/tslib/tslib.js:111:16)
2020-12-08T23:15:37: at RetryHandler.sendEventsWithRetry (/var/www/socialcomm/source/server/node_modules/@amplitude/node/dist/src/retryHandler.js:40:24) `

- SDK Version: 1.1.0
- Node version: 15.3.0