Only link symbols if there might be any

Summary:
fixes T8260. Only turn on symbol links if:

• The repository has any configuration about symbols, or
• There actually are symbols in the repository.

Test Plan: Look at revisions and files in various states of configurations and having symbols.

Reviewers: #blessed_reviewers, epriestley

Reviewed By: #blessed_reviewers, epriestley

Subscribers: joshuaspence, Korvin, epriestley

Maniphest Tasks: T8260

Differential Revision: https://secure.phabricator.com/D12946