Fix remove() to use null check instead of truthy check, update docblocks

- `if (\$key)` was changed to `if (\$key !== null)` to correctly handle
  empty string ('') and '0' keys, which are falsy but valid
- Updated `@param null \$key` docblocks to `@param string|null \$key`
  in both Segment and SegmentInterface

Addresses CodeRabbit review comments on PR #99.

Author: koriym

src/Segment.php

@@ -117,17 +117,20 @@ public function clear()
     /**
      * Remove a key from the segment, or remove the entire segment (including key) from the session
      *
-     * @param null $key
+     * @param string|null $key The key to remove, or null to clear the entire segment.
      */
     public function remove(?string $key = null) {
-        if ($this->resumeSession()) {
-            if($key){
-                if(isset($_SESSION[$this->name]) && array_key_exists($key, $_SESSION[$this->name])){
-                    unset($_SESSION[$this->name][$key]);
-                }
-            } else {
-                unset($_SESSION[$this->name]);
-            }
+        if (! $this->resumeSession()) {
+            return;
+        }
+        if ($key === null) {
+            unset($_SESSION[$this->name]);
+            unset($_SESSION[Session::FLASH_NOW][$this->name]);
+            unset($_SESSION[Session::FLASH_NEXT][$this->name]);
+            return;
+        }
+        if (array_key_exists($key, $_SESSION[$this->name])) {
+            unset($_SESSION[$this->name][$key]);
         }
     }
 

src/SegmentInterface.php

@@ -138,7 +138,7 @@ public function keepFlash();
     /**
      * Remove a key from the segment, or remove the entire segment (including key) from the session
      *
-     * @param null $key
+     * @param string|null $key The key to remove, or null to clear the entire segment.
      */
     public function remove(?string $key = null);
 }

tests/SegmentTest.php

@@ -194,6 +194,12 @@ public function testGetFlashDoesNotStartSession()
         $this->assertFalse($this->session->isStarted());
     }
 
+    public function testRemoveDoesNothingWhenSessionNotStarted(): void
+    {
+        $this->segment->remove('foo');
+        $this->assertFalse($this->session->isStarted());
+    }
+
     public function testRemoveKey(){
         $this->segment->set('foo', 'bar');
         $this->segment->set('baz', 'dib');