Clarification of nodeToString() use cases

Hi, Hackers!

In the AQO project (Adaptive Query Optimization) [1] the nodeToString()
function is used by the planner to convert an query parse tree into a
string to generate a hash value [2].
In PostgreSQL v.11 call nodeToString(parse) segfaulted.
The reason is: parse tree node for XMLNAMESPACES clause has null pointer
in the case of DEFAULT namespace (the pointer will be initialized at
executor on the first call). Function _outValue() uses value->val.str[0]
[3] without checking of value->val.str.

I want to know, which of next options is correct:
1. Converting a parse tree into string with nodeToString() is illegal
operation. We need to add a comment to the description of nodeToString().
2. We can use nodeToString() for parse tree convertation. In this case
we need to check node variable 'value->val.str' to NULL pointer (Now I
use this approach, see attachment).

[1] https://github.com/postgrespro/aqo
[2] hash.c, line 55.
[3] outfuncs.c, line 3312.

--
Andrey Lepikhov
Postgres Professional
https://postgrespro.com
The Russian Postgres Company