fix: Do not throw error when IF NOT EXISTS is used on ADD COLUMN (#2092)

Author: likejane

internal/endtoend/testdata/ddl_alter_table_add_column_if_not_exists/postgresql/pgx/v4/go/db.go

@@ -0,0 +1,2 @@
+-- name: Placeholder :exec
+SELECT 1;

internal/endtoend/testdata/ddl_alter_table_add_column_if_not_exists/postgresql/pgx/v4/go/models.go

@@ -0,0 +1,2 @@
+CREATE TABLE foo (bar text NOT NULL, baz text NOT NULL);
+ALTER TABLE foo ADD COLUMN IF NOT EXISTS bar text;

internal/endtoend/testdata/ddl_alter_table_add_column_if_not_exists/postgresql/pgx/v4/go/query.sql.go

@@ -0,0 +1,13 @@
+{
+  "version": "1",
+  "packages": [
+    {
+      "path": "go",
+      "engine": "postgresql",
+      "sql_package": "pgx/v4",
+      "name": "querytest",
+      "schema": "schema.sql",
+      "queries": "query.sql"
+    }
+  ]
+}

internal/endtoend/testdata/ddl_alter_table_add_column_if_not_exists/postgresql/pgx/v4/query.sql

@@ -0,0 +1,2 @@
+-- name: Placeholder :exec
+SELECT 1;