handle missing author in pull requests

```
error [TypeError: Cannot call method 'split' of undefined]
stack TypeError: Cannot call method 'split' of undefined
    at /Users/lalit/.nvm/v0.10.24/lib/node_modules/github-changes/bin/index.js:368:68
    at Array.forEach (native)
    at commitFormatter (/Users/lalit/.nvm/v0.10.24/lib/node_modules/github-changes/bin/index.js:316:8)
    at formatter (/Users/lalit/.nvm/v0.10.24/lib/node_modules/github-changes/bin/index.js:392:39)
    at /Users/lalit/.nvm/v0.10.24/lib/node_modules/github-changes/bin/index.js:453:35
    at tryCatch1 (/Users/lalit/.nvm/v0.10.24/lib/node_modules/github-changes/node_modules/bluebird/js/main/util.js:86:19)
    at Promise$_callHandler [as _callHandler] (/Users/lalit/.nvm/v0.10.24/lib/node_modules/github-changes/node_modules/bluebird/js/main/promise.js:681:13)
    at Promise$_settlePromiseFromHandler [as _settlePromiseFromHandler] (/Users/lalit/.nvm/v0.10.24/lib/node_modules/github-changes/node_modules/bluebird/js/main/promise.js:697:18)
    at Promise$_settlePromiseAt [as _settlePromiseAt] (/Users/lalit/.nvm/v0.10.24/lib/node_modules/github-changes/node_modules/bluebird/js/main/promise.js:879:14)
    at Promise$_fulfillPromises [as _fulfillPromises] (/Users/lalit/.nvm/v0.10.24/lib/node_modules/github-changes/node_modules/bluebird/js/main/promise.js:981:14)
```