Handle the changed behavior of Parser::Base#tokenize at 2.0.0.beta6

It may return nil for AST even though there are no syntax errors.
e.g. sources which contain only comments

This change makes RuboCop always abort further inspection
if there are syntax errors.
Add nil guard for some cops
Mark a spec as broken with Parser 2.0.0.beta6

The spec is broken with Parser 2.0.0.beta6, would be fixed with beta7.
https://github.com/whitequark/parser/commit/8b066bf
Merge pull request #291 from yujinakayama/fix-broken-specs-with-parser2.0.0beta6

Fix broken specs with Parser 2.0.0.beta6