On the first `foreach` loop in Text_Diff::_check()`, `$prevtype` is `null`. As `instanceof` requires the class name term to be an object or string, a fatal error is thrown:
>Fatal error: Uncaught Error: Class name must be a valid object or a string on line 279
This change:
* Adds a simple test for the `Text_Diff::_check()` method, which is how the bug was discovered as the test could never pass with the code as-is.
* Adds a defensive guard to protect against the fatal. It checks if `$prevtype` is not `null` as a pre-condition to for checking the instance. This bugfix also resolves the failing test.
Follow-up to [49194], [7747].
Props jrf, hellofromTonya.
See #62083.
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