Handle undefined text area values

Check to see if a text area's `val` is defined before trying to call
`replace()` on it.

This was causing failures in RSpec tests; here's a snippet of output from `bundle exec rspec spec/features/notes_on_merge_requests_spec.rb`:

    Failures:

      1) Comments On a merge request the note form should be valid
         Failure/Error: visit project_merge_request_path(project, merge_request)
         Capybara::Poltergeist::JavascriptError:
           One or more errors were raised in the Javascript code on the page. If you don't care about these errors, you can ignore them by setting js_errors: false in your Poltergeist configuration (see documentation for details).

           TypeError: 'undefined' is not an object (evaluating 'val.replace')
           TypeError: 'undefined' is not an object (evaluating 'val.replace')
               at http://127.0.0.1:58906/assets/application.js:76468
               at http://127.0.0.1:58906/assets/application.js:76475
               at http://127.0.0.1:58906/assets/application.js:74482 in MergeRequest
               at http://127.0.0.1:58906/namespace1/gitlabhq/merge_requests/1:553
         # ./spec/features/notes_on_merge_requests_spec.rb:13:in `block (3 levels) in <top (required)>'