dolphin: Add ISNULL function signature

Fixes #630

Author: kyleconroy

internal/engine/dolphin/stdlib.go

@@ -707,9 +707,13 @@ func defaultSchema(name string) *catalog.Schema {
 			ReturnType: &ast.TypeName{Name: "any"},
 		},
 		{
-			Name:       "ISNULL",
-			Args:       []*catalog.Argument{},
-			ReturnType: &ast.TypeName{Name: "any"},
+			Name: "ISNULL",
+			Args: []*catalog.Argument{
+				{
+					Type: &ast.TypeName{Name: "any"},
+				},
+			},
+			ReturnType: &ast.TypeName{Name: "bool"},
 		},
 		{
 			Name:       "IS_FREE_LOCK",