Allow safe-navigation in ternary conditions without parenthesis

[timon]

When enforced style is require_parenthesis_if_complex, the following
code is considered valid:

  foo = bar.baz? ? a : b

But safe navigation operator produces an offence.
This commit makes the following code also acceptable:

  foo = bar&.baz ? a : b

---

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[koic]

It is difficult to define simple condition and complex condition. It may be better for offense to occur because foo&.bar is a conditional expression similar to foo && bar.
But I think this PR is valid because foo[:bar] has no offense. It is also shown as simple condition in the test cases as a symbolic syntax.

[bbatsov]

Thanks!