BUG #15101: function set search_path = '' breaks dump/restore

The following bug has been logged on the website:

Bug reference:      15101
Logged by:          Andrew Gierth
Email address:      andrew@tao11.riddles.org.uk
PostgreSQL version: 10.3
Operating system:   any
Description:

Given:

create or replace function foo()
 returns text language sql
 as $$
  select current_schemas(true)::text;
 $$
 set search_path = '';

dump/restore (or \ef) fails with:

ERROR:  zero-length delimited identifier at or near """"

The problem obviously being that pg_get_functiondef is returning invalid
syntax for the search_path setting.