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.

tests/BoundaryTest.php

@@ -35,6 +35,11 @@ it('length with boundaries', function ($expectedAmount, Period $period) {
     expect($period->length())->toEqual($expectedAmount);
 })->with('periods_with_amounts_of_included_dates');
 
+it('length is zero when excluding boundaries leaves nothing', function (Period $period) {
+    expect(iterator_count($period))->toEqual(0);
+    expect($period->length())->toEqual(0);
+})->with('periods_left_empty_by_excluded_boundaries');
+
 it('overlap with excluded boundaries', function () {
     $a = Period::make('2018-01-01', '2018-01-05', boundaries: Boundaries::EXCLUDE_END());
     $b = Period::make('2018-01-05', '2018-01-10');