Add a check before using ajax_page_state

Even though the changes are pretty small (+10/-2) I think they are easier to understand as a series of small commits, so I opened a MR for this issue.

I confess that I have not tested this at all, except that I installed the module and did not get a WSOD. Testing is the reviewer's job, right? ;)

Good points in #4.

I did the easy part, adding a check for !empty(...) before using the POST variable. If the check is to provide any protection, we have to ignore the jQuery version in the AJAX request unless it comes with a token.

This means we have to invalidate any cached pages that have set jquery_version but not jquery_version_token in Drupal.settings. Can we just advise site owners to clear caches when upgrading to the next release, or do we have to write an update function?

I may not have time today to add anything to the tests. I do not see any AJAX tests in the module, but maybe I can use ajax.test in the core simpletest module as a guide.

I struggled for a while, trying to come up with an integration test. If you really want, then I can keep trying.

What I have now is more like a unit test. It directly calls jquery_update_library_alter() three times: the second time, it sets jquery_version in a POST variable, and the third time it also sets a token in a POST variable.

    $libraries = drupal_get_library('system');
    // ... assertions ...
    jquery_update_library_alter($libraries, 'system');
    // ... assertions ...

That is not ideal. Since this is really a functional test, not a unit test, the alter function is already applied in the first line. So these assertions are redundant. I could leave out the second set of assertions, or I could replace the first line with an explicit array. The second option would make it more like a unit test.

Maybe, instead of altering the same $libraries variable three times (since it is passed by reference to the alter function) I should do something like

    $original_libraries = drupal_get_library('system');
    $libraries = $original_libraries;
    // ... assertions ...
    $libraries = $original_libraries;
    jquery_update_library_alter($libraries, 'system');
    // ... assertions ...

I am attaching the new commit as a test-only patch. I expect it to fail when applying the alter function with just jquery_version set in the POST variable (two assertions).

I broke a long line in order to keep CodeSniffer happy.

Here is a version of the test-only patch that includes that fix and also removes some WIP changes that I accidentally included in #6.

The testbot will set this issue back to NW because of the test-only patch.