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.
Merge pull request #561 from stof/webassert_usage

Replaced most null safeguards in tests with usages of WebAssert