Hover is not unpinned even when it is empty

Created by: lguychard

Steps to reproduce:

1. Sync https://bitbucket.sgdev.org/projects/SOUR/repos/vegeta to a Sourcegraph instance.
2. Navigate to http://localhost:3080/bitbucket.sgdev.org/SOUR/vegeta/-/blob/lib/targets.go#L137:17, with basic code intel activated.
3. Click on Lexer at line 137.
4. Hover jlexer at line 137, without clicking.

Expected behavior:

Hover overlay appears for jlexer:

Actual behavior:

No hover overlay appears for jlexer, or any other symbol, unless you unpin the hover overlay.

This is because getHoverActions() emits an empty array instead of null when no hover actions are contributed, which trips up codeintellify's unpinning logic (rel https://github.com/sourcegraph/codeintellify/issues/156)