Avoid using list_length() to test for empty list.

The standard way to check for list emptiness is to compare the
List pointer to NIL; our list code goes out of its way to ensure
that that is the only representation of an empty list.  (An
acceptable alternative is a plain boolean test for non-null
pointer, but explicit mention of NIL is usually preferable.)

Various places didn't get that memo and expressed the condition
with list_length(), which might not be so bad except that there
were such a variety of ways to check it exactly: equal to zero,
less than or equal to zero, less than one, yadda yadda.  In the
name of code readability, let's standardize all those spellings
as "list == NIL" or "list != NIL".  (There's probably some
microscopic efficiency gain too, though few of these look to be
at all performance-critical.)

A very small number of cases were left as-is because they seemed
more consistent with other adjacent list_length tests that way.

Peter Smith, with bikeshedding from a number of us

Discussion: https://postgr.es/m/CAHut+PtQYe+ENX5KrONMfugf0q6NHg4hR5dAhqEXEc2eefFeig@mail.gmail.com

src/backend/commands/statscmds.c

@@ -339,7 +339,7 @@ CreateStatistics(CreateStatsStmt *stmt)
 	if ((list_length(stmt->exprs) == 1) && (list_length(stxexprs) == 1))
 	{
 		/* statistics kinds not specified */
-		if (list_length(stmt->stat_types) > 0)
+		if (stmt->stat_types != NIL)
 			ereport(ERROR,
 					(errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
 					 errmsg("when building statistics on a single expression, statistics kinds may not be specified")));
@@ -391,7 +391,7 @@ CreateStatistics(CreateStatsStmt *stmt)
 	 * automatically. This allows calculating good estimates for stats that
 	 * consider per-clause estimates (e.g. functional dependencies).
 	 */
-	build_expressions = (list_length(stxexprs) > 0);
+	build_expressions = (stxexprs != NIL);
 
 	/*
 	 * Check that at least two columns were specified in the statement, or