Fix wrong results returned by isEnabled

When language filter plugin is disabled and language filter is enabled in code using JFactory::getApplication()->setLanguageFilter(true) calling JLanguageAssociations->isEnabled causes notices and fatal errors and returns wrong result if item associations are in fact disabled.

What happens is that JPluginHelper::getPlugin returns empty array when certain plugin is disabled and since there is no params to be passed to JRegistry and $enabled gets default value.
Missing object property "language".
[bug] [32258] FOFModel::getTmpInstance() clears input before getItemList() is run
Close #2227
Correcting error in sr-YU
[bug] [32295] Add gradient for background at the login screen for IE10
Close #2321
Generated CSS from LESS using build/generatecss.php
[imp] [33641] Add time unit in cache settings description
Close #3499
[bug] [33701] User Notes Categories - Error in menu and filter selection
Close #3559
Fixed missing space in toolbar buttons (type modal)
Merge pull request #3572 from nonumber/patch-1

Fixed missing space in toolbar buttons (type modal)
[bug] [33413] Cannot upload file to sub-folder in Media Manager in back end
Close #3575
[bug] [32957] Removing doubled description tab in template styles
Close #3049
[bug] [33625] Replace "bootstrap_size":"1" with "bootstrap_size":"0"
Close #3470
[bug] [33600] Remove validate-jquery-uncompressed.js
Close #3426
[imp] [32323] Removes excessive padding-top on the body element when the viewport is at 738px wide
Close #2650
[#32862] Removed // comments from bootstrap css.

The // comments are not intended to be present in the css files.  They are
only present in the .less files to identify changes from standard bootstrap.
Using "joomla.content.icons" JLayout for article and feature view
Adding cy_GB (Welsh) language to installation
Add link to coding standards to contributing.md
Close #3593
Merge pull request #3584 from pjwiseman/32862

[#32862] Removed // comments from bootstrap css.