For non-void functions, forbid bare return

[quasilyte]

Code Example

/**
 * @return string|null
 */
function f($x) {
  if (!$x) {
    return; // Better to write as "return null"
  }
  // ...
}

Actual Behavior

No warnings.

Expected Behavior

Suggest explicit return null. Functions with void results should be ignored.

[i582]

Will be tracked further in #1123.