fix: return 0 from length() when excluded boundaries leave a period empty

Excluding the boundaries of a one or two unit long period pushes the
included start past the included end. diff()->days is unsigned and the
HOUR/MINUTE/SECOND branch applied abs() outright, so that backwards range
was measured as a positive distance and the unconditional +1 inflated it
further: a one day period reported 1, 2, 2 and 3 for EXCLUDE_NONE, START,
END and ALL, while iterating the same objects yielded nothing.

Return 0 up front when the included start is past the included end. This
matches what the MONTH and YEAR precisions already returned through
iterator_count(), and lets the now unreachable abs() go.

src/PeriodTraits/PeriodGetters.php

@@ -66,6 +66,13 @@ trait PeriodGetters
 
     public function length(): int
     {
+        // Excluding the boundaries of a period that is only a few units long can push the
+        // included start past the included end. Such a period contains nothing, which is what
+        // iterating over it yields, so its length is 0 rather than a distance measured backwards.
+        if ($this->includedStart() > $this->includedEnd()) {
+            return 0;
+        }
+
         // Length of month and year are not fixed, so we can't predict the length without iterate
         // TODO: maybe we can use cal_days_in_month ?
         if ($this->precision->equals(Precision::MONTH(), Precision::YEAR())) {
@@ -73,7 +80,7 @@ trait PeriodGetters
         }
 
         if ($this->precision->equals(Precision::HOUR(), Precision::MINUTE(), Precision::SECOND())) {
-            $length = abs($this->includedEnd()->getTimestamp() - $this->includedStart()->getTimestamp());
+            $length = $this->includedEnd()->getTimestamp() - $this->includedStart()->getTimestamp();
 
             if ($this->precision->equals(Precision::SECOND())) {
                 return $length + 1;