explain: fix decoding plan gist with CALL stmts

Previously we'd hit a nil pointer error when trying to print `Proc`
argument when it is `nil`. The problem is that it is stored as
`*tree.RoutineExpr`, and `(*tree.RoutineExpr)(nil)` is different from
`(tree.TypedExpr)(nil)` that we check for (and short-circuit on) in the
`Expr` call.

I have an idea for how we could've caught this, but it'll be done in a
separate commit.

Release note (bug fix): CockroachDB would previously encounter an
internal error when decoding plan gists of the plans with CALL
statements. The bug has been present since 23.2 version and is now
fixed.

Author: yuzefovich

pkg/sql/opt/exec/execbuilder/testdata/explain_gist

@@ -202,3 +202,9 @@ query T
 SELECT crdb_internal.decode_external_plan_gist('Aifvzn5p':::STRING)
 ----
 • create view
+
+# Regression test for hitting an internal error on CALL expressions (#143211).
+query T nosort
+SELECT crdb_internal.decode_external_plan_gist('Aj0=':::STRING)
+----
+• call

pkg/sql/opt/exec/explain/emit.go

@@ -1015,7 +1015,14 @@ func (e *emitter) emitNodeAttributes(n *Node) error {
 
 	case callOp:
 		a := n.args.(*callArgs)
-		ob.Expr("procedure", a.Proc, nil /* columns */)
+		if a.Proc != nil {
+			// Unlike other expressions, we store Proc as *tree.RoutineExpr, so
+			// the nil value in Proc is different from the nil value that is
+			// checked in Expr:
+			//  (*tree.RoutineExpr)(nil)  vs (tree.TypedExpr)(nil)
+			// so we need an explicit nil check.
+			ob.Expr("procedure", a.Proc, nil /* columns */)
+		}
 
 	case simpleProjectOp,
 		serializingProjectOp,