This fixes a problem noted as a followup to bug #8648: if a query has a semantically-empty target list, e.g. SELECT * FROM zero_column_table, ruleutils.c will dump it as a syntactically-empty target list, which was not allowed. There doesn't seem to be any reliable way to fix this by hacking ruleutils (note in particular that the originally zero-column table might since have had columns added to it); and even if we had such a fix, it would do nothing for existing dump files that might contain bad syntax. The best bet seems to be to relax the syntactic restriction.
Also, add parse-analysis errors for SELECT DISTINCT with no columns (after *-expansion) and RETURNING with no columns. These cases previously produced unexpected behavior because the parsed Query looked like it had no DISTINCT or RETURNING clause, respectively. If anyone ever offers a plausible use-case for this, we could work a bit harder on making the situation distinguishable.
Arguably this is a bug fix that should be back-patched, but I'm worried that there may be client apps or PLs that expect "SELECT ;" to throw a syntax error. The issue doesn't seem important enough to risk changing behavior in minor releases.
This commit introduces another bug:
# create table colours (id serial, name text, visible boolean);
CREATE TABLE
# insert into colours (name, visible) values ('blue',true),('yellow',true),('ultraviolet',false),('green',true),('infrared',false);