Allow title to be empty if specifying path for the identifier

[markphilpot]

Describe the bug
Confluence of assumptions:

• In backend.js an identifier is required even if {{slug}} is not used in the config.yml::collections:slug

• IDENTIFIER_FIELDS includes title and path but searches for title first. So if you specify both title and path as fields, title is always preferred.

• For a microblog, the title needs to be an empty string in the rss.

In my configuration, I set title to an empty string, and path to an identifier, but I always get Collection must have a field name that is a valid entry identifier error because title is empty.

I'm not sure what the right solution is given how the logic is distributed, but ideally it would support title as an empty string with path specified as the identifier. Or more basically, only require the identifier field if {{slug}} is in the slug template string

[erquhart]

Once #1543 merges, you'll be able to explicitly set "path" (or any other field name) as the identifier.

[erquhart]

Fixed in #1543.