Removed accidental, incorrectly commited file contents.

It appears that the contents of `InstantiateByStringAtPropertyTest.php`
was somehow replaced with the contents of another file, and before the
first commit of this file to the repo. So no history exists.

The test was simple: can we instantiate an object by doing:

  ```
  $obj = new $this->some_string
  ```
Added `UndefinedPropertiesAreNullTest.php`