make AssertEmptyIsDiscouragedRule autofixable #338

@mitelg looking at the "errors" I had to fix because of your recent PR made me think whether this new rule is a good candidate for a auto-fixer rule, similar as done in #252

this means in case we e.g. know a $errors type beeing used in $this->assertEmpty($errors);/$this->assertNotEmpty($errors); is a array fix it to $this->assertCount(0, $errors); or $this->assertNotCount(0, $errors) etc.

in case we don't know the type, we should not touch it, but for the most common types we should be able to autofix it