Replaced most null safeguards in tests with usages of WebAssert

The ``assertNotNull`` calls are kept in 2 cases:
- in the ``TraversingTest`` as they are the real assertions of the tests
- in case ``findById()`` or ``findButton()`` are used, as these shortcut are more readable than calling elementExists() with a named selector, especially considering the fact that it would require applying the XPath escaping manually in these places.