check length in _.first and _.last, fix #2495

[xcodebuild]

Fix #2495

Now return undefined when use _.first or _.last on empty array.

Array.prototype[0] = 'boo';
_.first([]) // undefined

var arr = [];
arr[-1] = 'boo'
_.last(arr); // undefined

[coveralls]

Coverage remained the same at 96.867% when pulling 507b858 on CodeFalling:check-length-in-first-last into 9c18116 on jashkenas:master.

[jridgewell]

Thanks!

[psixdev]

I would like to understand the motivation for this change. Why in this case first should return undefined instead of boo? Moreover, this does not work completely for sparse arrays: