`IS NOT DISTINCT FROM` inside an EXISTS subquery panics: nil pointer in `IsNotDistinctFrom.String`

[johannesdb]

Version: reproduced on dolthub/doltgresql:1.1.0 and 1.0.0 (clean Docker container, empty database; latest run 2026-08-15)
Class: internal panic (recovered; the query fails, connection survives)

Repro

CREATE TABLE t_indf (id int PRIMARY KEY, v text);
INSERT INTO t_indf VALUES (1, 'a'), (2, NULL);

-- top level works on both engines:
SELECT id, v IS NOT DISTINCT FROM 'a' FROM t_indf ORDER BY id;

-- inside an EXISTS subquery it panics:
SELECT count(*) FROM t_indf o
WHERE EXISTS (SELECT 1 FROM t_indf i WHERE i.v IS NOT DISTINCT FROM o.v);

Actual (DoltgreSQL 1.1.0)

ERROR:  receiveMessage recovered panic: runtime error: invalid memory address or nil pointer dereference
...
github.com/dolthub/doltgresql/server/expression.(*IsNotDistinctFrom).String(...)
	github.com/dolthub/doltgresql/server/expression/is_not_distinct_from.go:95
github.com/dolthub/vitess/go/vt/sqlparser.InjectedExpr.Format(...)

The panic fires in the expression's String() while the plan builder stringifies the
subquery (planbuilder.(*Builder).buildSubquery → sqlparser.String), i.e. before
execution. (Full stack trace available.)

Expected (PostgreSQL 15, same probe)

 matched → 2

---

Part of a clean-room deviation battery (fresh official Docker container + empty
database per probe, postgres:15 control running identical probes). Filing the
findings together so they can be triaged/batched as a set — index in a comment on
#2600. Happy to re-test any of them against a nightly/branch build.