Allow empty target list in SELECT.

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.

src/backend/parser/parse_clause.c

@@ -2011,6 +2011,20 @@ transformDistinctClause(ParseState *pstate,
 									  true);
 	}
 
+	/*
+	 * Complain if we found nothing to make DISTINCT.  Returning an empty list
+	 * would cause the parsed Query to look like it didn't have DISTINCT, with
+	 * results that would probably surprise the user.  Note: this case is
+	 * presently impossible for aggregates because of grammar restrictions,
+	 * but we check anyway.
+	 */
+	if (result == NIL)
+		ereport(ERROR,
+				(errcode(ERRCODE_SYNTAX_ERROR),
+				 is_agg ?
+				 errmsg("an aggregate with DISTINCT must have at least one argument") :
+				 errmsg("SELECT DISTINCT must have at least one column")));
+
 	return result;
 }
 
@@ -2115,6 +2129,11 @@ transformDistinctOnClause(ParseState *pstate, List *distinctlist,
 									  true);
 	}
 
+	/*
+	 * An empty result list is impossible here because of grammar restrictions.
+	 */
+	Assert(result != NIL);
+
 	return result;
 }