Let's look at evaluate_COUNT() and the following lines in particular:

if (NULL != arg2 && '\0' == *arg2 && (0 != numeric_search || OP_LIKE == op))
	zbx_free(arg2);

...

for (i = 0; i < values.values_num; i++)
{
	if (NULL == arg2 || ...)
	{
		count++;
	}
}

These lines mean that if we are doing a count() on numeric items and the second argument is empty, we will always count existing values.

For instance, the following triggers will always fire, regardless of item value:

{Zabbix server:numeric.count(#1,,eq)} <> 0
{Zabbix server:numeric.count(#1,,lt)} <> 0