Error "Undefined array key 0" fix

There's an error, if you pass an empty PeriodCollection to `overlap()` or `subtract()` method since there's no check whether the collection actually has any values.

Steps to reproduce:
```php

        $period = Period::make('2021-12-31', '2022-01-01');
        $collection = new PeriodCollection;

        $period->overlap(... $collection); // Throws ErrorException "Undefined array key 0"
        $period->subtract(... $collection); // Throws ErrorException "Undefined array key 0"
```

P.S. Thank you for your products and Merry Christmas! :)

src/PeriodTraits/PeriodOperations.php

@@ -38,7 +38,9 @@ public function gap(Period $period): ?static
 
     public function overlap(Period ...$others): ?static
     {
-        if (count($others) > 1) {
+        if (count($others) === 0) {
+            return null;
+        } else if (count($others) > 1) {
             return $this->overlapAll(...$others);
         } else {
             $other = $others[0];
@@ -115,7 +117,9 @@ public function overlapAny(Period ...$others): PeriodCollection
      */
     public function subtract(Period ...$others): PeriodCollection
     {
-        if (count($others) > 1) {
+        if (count($others) === 0)
+            return PeriodCollection::make($this);
+        } else if (count($others) > 1) {
             return $this->subtractAll(...$others);
         } else {
             $other = $others[0];