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/utils/adt/jsonb_gin.c

@@ -567,7 +567,7 @@ extract_jsp_path_expr(JsonPathGinContext *cxt, JsonPathGinPath path,
 	/* extract a list of nodes to be AND-ed */
 	List	   *nodes = extract_jsp_path_expr_nodes(cxt, path, jsp, scalar);
 
-	if (list_length(nodes) <= 0)
+	if (nodes == NIL)
 		/* no nodes were extracted => full scan is needed for this path */
 		return NULL;
 

src/backend/utils/adt/jsonpath_exec.c

@@ -2552,7 +2552,7 @@ JsonValueListLength(const JsonValueList *jvl)
 static bool
 JsonValueListIsEmpty(JsonValueList *jvl)
 {
-	return !jvl->singleton && list_length(jvl->list) <= 0;
+	return !jvl->singleton && (jvl->list == NIL);
 }
 
 static JsonbValue *

src/backend/utils/adt/jsonpath_gram.y

@@ -459,7 +459,7 @@ makeIndexArray(List *list)
 	ListCell   *cell;
 	int			i = 0;
 
-	Assert(list_length(list) > 0);
+	Assert(list != NIL);
 	v->value.array.nelems = list_length(list);
 
 	v->value.array.elems = palloc(sizeof(v->value.array.elems[0]) *

src/backend/utils/adt/ruleutils.c

@@ -8114,7 +8114,7 @@ get_parameter(Param *param, deparse_context *context)
 				{
 					deparse_namespace *dpns = lfirst(lc);
 
-					if (list_length(dpns->rtable_names) > 0)
+					if (dpns->rtable_names != NIL)
 					{
 						should_qualify = true;
 						break;

src/backend/utils/adt/selfuncs.c

@@ -3408,7 +3408,7 @@ estimate_num_groups_incremental(PlannerInfo *root, List *groupExprs,
 	 * for normal cases with GROUP BY or DISTINCT, but it is possible for
 	 * corner cases with set operations.)
 	 */
-	if (groupExprs == NIL || (pgset && list_length(*pgset) < 1))
+	if (groupExprs == NIL || (pgset && *pgset == NIL))
 		return 1.0;
 
 	/*

src/backend/utils/adt/tsquery.c

@@ -829,7 +829,7 @@ parse_tsquery(char *buf,
 
 	close_tsvector_parser(state.valstate);
 
-	if (list_length(state.polstr) == 0)
+	if (state.polstr == NIL)
 	{
 		ereport(NOTICE,
 				(errmsg("text-search query doesn't contain lexemes: \"%s\"",