Fixed NPE on quick assist proposals

In the former implementation, the implicit conversion `Predef.refArrayOps`
could be applied on a `null` value, because `null` was returned if
`corrections.isEmpty` was true, leading to a NPE.

Fix #1002014
Merge pull request #618 from dotta/issue/npe-on-quickassist-1002014

Fixed NPE on quick assist proposals