fix: empty search_path (#48151)

## TL;DR
Restores handling for functions with `search_path` set to `''` editing
them in the UI was failing with a Postgres `zero-length delimited
identifier` error since the SafeSql refactor dropped the empty-string
sentinel conversion

## ref
- closes #48149


<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->

## Summary by CodeRabbit

* **Bug Fixes**
* Preserved empty `search_path` configuration values when updating
database functions.
* Prevented empty configuration values from being altered or lost during
function updates.

* **Tests**
* Added coverage verifying that function definitions can be updated
without changing an existing empty `search_path` setting.

<!-- end of auto-generated comment: release notes by coderabbit.ai -->

packages/pg-meta/src/pg-meta-functions.ts

@@ -260,7 +260,7 @@ function _generateCreateFunctionSql(
           Object.entries(config_params).map(([param, value]) =>
             value === 'FROM CURRENT'
               ? safeSql`SET ${qualifiedIdent(param)} FROM CURRENT`
-              : safeSql`SET ${qualifiedIdent(param)} TO ${value}`
+              : safeSql`SET ${qualifiedIdent(param)} TO ${value === '""' ? literal('') : value}`
           ),
           '\n'
         )

packages/pg-meta/test/functions.test.ts

@@ -349,6 +349,38 @@ withTestDatabase('create function with various config_params values', async ({ e
   await executeQuery(removeSql1)
 })
 
+withTestDatabase('update function with empty string search_path', async ({ executeQuery }) => {
+  const { sql: createSql } = pgMeta.functions.create({
+    name: 'test_func_empty_search_path',
+    schema: 'public',
+    definition: 'select 1',
+    return_type: safeSql`integer`,
+    language: 'sql',
+    config_params: { search_path: safeSql`''` },
+  })
+  await executeQuery(createSql)
+
+  const { sql: retrieveSql, zod: retrieveZod } = pgMeta.functions.retrieve({
+    name: 'test_func_empty_search_path',
+    schema: 'public',
+    args: [],
+  })
+  const result = retrieveZod.parse((await executeQuery(retrieveSql))[0])
+  expect(result!.config_params).toEqual({ search_path: '""' })
+
+  const { sql: updateSql } = pgMeta.functions.update(asSavedFunction(result!), {
+    definition: 'select 2',
+  })
+  await executeQuery(updateSql)
+
+  const resultUpdated = retrieveZod.parse((await executeQuery(retrieveSql))[0])
+  expect(resultUpdated!.definition).toBe('select 2')
+  expect(resultUpdated!.config_params).toEqual({ search_path: '""' })
+
+  const { sql: removeSql } = pgMeta.functions.remove(asSavedFunction(resultUpdated!))
+  await executeQuery(removeSql)
+})
+
 withTestDatabase(
   'create function with namespaced custom GUC config_params',
   async ({ executeQuery }) => {