Fix missing explicit nullable values

Author: sunkan

src/BasicDocument.php

@@ -44,7 +44,7 @@ public function has(?string $path): bool
 		}
 	}
 
-	public function get(string $path = null): mixed
+	public function get(?string $path = null): mixed
 	{
 		return $this->reference($path ?? '')->getValue();
 	}

src/Document.php

@@ -6,12 +6,12 @@ interface Document
 {
 	public function getId(): string;
 
-	public function has(string $path): bool;
+	public function has(?string $path): bool;
 
 	/**
 	 * @return ($path is null ? array<string, mixed> : mixed)
 	 */
-	public function get(string $path = null): mixed;
+	public function get(?string $path = null): mixed;
 
 	public function findPathToParent(string $field, mixed $searchValue): string;
 

src/DocumentCollection.php

@@ -61,7 +61,7 @@ public function has(?string $path): bool
 		return false;
 	}
 
-	public function get(string $path = null): mixed
+	public function get(?string $path = null): mixed
 	{
 		foreach ($this->documents as $document) {
 			if ($document->has($path)) {