Standardized usage of undefined - saves 12 gz bytes

When checking for `undefined` variables, the vast majority of the jQuery codebase does so by checking  for the `undefined` value. A few outliers still used `typeof`, when it wasn't necessary. By making a few updates, the code became more consistent, and some bytes were freed up.

```
Running "compare_size:files" (compare_size) task
   raw     gz Sizes
246507  73045 dist/jquery.js
 84289  29471 dist/jquery.min.js

   raw     gz Compared to master @ d837f119c3729565103005d5d7fa89e1dd8110cb
   +12     -7 dist/jquery.js
   -10    -11 dist/jquery.min.js
```

Cheers,
Chris
In this case, `!` would be clearer