after() throws nonsense error if called on an empty selection

[matteosistisette]

Consider this expression:

$("#nonexistent").after(",");

If there is at least one element with id="nonexistent", then this expression adds a comma after it.

If there is no such element, it throws the following error: "Error: uncaught exception: Syntax error, unrecognized expression: ,"

I'm not sure whether it should throw any error at all (i think it shouldn't, should just do nothing), but _certainly_ this is not the error that should be thrown. The expression "," is perfectly valid, as it is an html string; the proof is that when the selection is not empty, it works.

[snover]

Duplicate of #7623.