Fix a crash bug in case debug_query_string is NULL

Fix a crash bug in case debug_query_string is NULL

pg_hint_plan believed that debug_query_string cannot be null when
parse_analyze is called, but for example in the case under
exec_describe_statement_message, it is not. We see the query string in
pstate even in the case, so use it instead in the case. Since pstate
is storing the query of the lowermost level, we should use
debug_query_string in other cases.