Fix slice and sliceSelf when collection is empty

src/Collections/Collection.php

@@ -126,9 +126,13 @@ class Collection extends ImmutableCollection implements ICollection {
      * @inheritDoc
      */
     public function sliceSelf (int $offset, ?int $length = null): static {
+        if ($this->count() === 0) {
+            $this->elements = [];
+        }
+        else {
             $this->_checkOffset($offset);
-
             $this->elements = array_slice($this->elements, $offset, $length, true);
+        }
         return $this;
     }
 

src/Collections/ImmutableCollection.php

@@ -289,9 +289,12 @@ class ImmutableCollection implements IImmutableCollection {
      * @inheritDoc
      */
     public function slice (int $offset, ?int $length = null): static {
-        $this->_checkOffset($offset);
-
         $output = new static();
+        if ($this->count() === 0) {
+            return $output;
+        }
+
+        $this->_checkOffset($offset);
 
         $currentIndex = 0;
         foreach ($this->elements as $key => $value) {