refactor: do not fail Rust deserialization when search_path is empty

sql/load_sql_context.sql

@@ -34,7 +34,12 @@ schemas_(oid, name) as (
 select
     jsonb_build_object(
         'config', jsonb_build_object(
-            'search_path', (select array_agg(schema_oid) from search_path_oids),
+            -- array_agg returns null when the search path is empty. Coalesce so
+            -- the context still deserializes into a (empty) sequence. Issue #650
+            'search_path', coalesce(
+                (select array_agg(schema_oid) from search_path_oids),
+                '{}'::oid[]
+            ),
             'role', current_role,
             'schema_version', graphql.get_schema_version()
         ),

test/expected/issue_650.out

@@ -0,0 +1,24 @@
+begin;
+    create table test_table (
+        id serial primary key,
+        name text not null,
+        description text
+    );
+    comment on schema public is e'@graphql({"introspection": true})';
+    -- baseline: a non-empty search_path resolves fine
+    set local search_path to public;
+    select graphql.resolve('{ __typename }');
+              resolve              
+-----------------------------------
+ {"data": {"__typename": "Query"}}
+(1 row)
+
+    -- an empty search_path should also resolve, with no visible schemas
+    set local search_path to '';
+    select graphql.resolve('{ __typename }');
+              resolve              
+-----------------------------------
+ {"data": {"__typename": "Query"}}
+(1 row)
+
+rollback;

test/sql/issue_650.sql

@@ -0,0 +1,19 @@
+begin;
+
+    create table test_table (
+        id serial primary key,
+        name text not null,
+        description text
+    );
+
+    comment on schema public is e'@graphql({"introspection": true})';
+
+    -- baseline: a non-empty search_path resolves fine
+    set local search_path to public;
+    select graphql.resolve('{ __typename }');
+
+    -- an empty search_path should also resolve, with no visible schemas
+    set local search_path to '';
+    select graphql.resolve('{ __typename }');
+
+rollback;