Fix #12357: be more kind to non-browser environments

[gibson042]

Sizes - compared to master @ d57363e

    265210        (+6)  dist/jquery.js                                         
     93442        (+4)  dist/jquery.min.js                                     
     33432        (+1)  dist/jquery.min.js.gz

[mikesherov]

Since we can't test this, can we at least add a comment to the code explaining this change?

[dmethvin]

I need to be more curious. Remember c79a533 ? That was the "useless guard" that just got put back in, and I remember thinking, "Huh, wonder why that was there." Well if I had just looked at the blame and seen dc83072 it would have been clear.