* Fix variable editor name-input bug

You couldn't delete an invalid character after typing it into the name-input field.
While investigating the issue turned out to be bigger, as there was a problem with
valid characters too. (See test scenarios below)

The fix seems to be, to remove an unnecessary check in the `changeVariableName`
action. There is theoretically now the possibility, that the `changeVariableName`
action is called with the same name, as the variable is already, but practically
there seems no possibility, that this could happen. A test, which checks that, had
to be removed too.

Test scenarios:
* 1st Scenario
    1. Type "@"
    2. Try deleting it
* 2nd Scenario
    1. Type "w"
    2. delete "w"
    3. Try typing "w" again

Fixes #26562

* Fix bug when updating existing variable