docs: Fix join filters with empty string

[ditatompel]

The default join() function will concatenating all of the filters value with commas instead of empty string. If more than one column filters parameter is given, the fetch query become something like:

id=1,&title=query,&completed=query

Giving join separator parameter with empty string fix the issue.

[vincjo]

I learned something. Thank you so much for your help

Just published in 1.14.5