fix(files): Never return a null ETag in DAV

Signed-off-by: provokateurin <kate@provokateurin.de>

lib/private/Files/Storage/Common.php

@@ -391,7 +391,7 @@ abstract class Common implements Storage, ILockingStorage, IWriteStreamStorage {
 	 * get the ETag for a file or folder
 	 *
 	 * @param string $path
-	 * @return string
+	 * @return string|false
 	 */
 	public function getETag($path) {
 		return uniqid();

lib/private/Files/Storage/DAV.php

@@ -721,10 +721,9 @@ class DAV extends Common {
 		return $stat ? $stat['permissions'] : 0;
 	}
 
-	/** {@inheritdoc} */
 	public function getETag($path) {
 		$meta = $this->getMetaData($path);
-		return $meta ? $meta['etag'] : null;
+		return $meta ? $meta['etag'] : false;
 	}
 
 	/**

lib/private/Files/Storage/Local.php

@@ -513,17 +513,11 @@ class Local extends \OC\Files\Storage\Common {
 		return true;
 	}
 
-	/**
-	 * get the ETag for a file or folder
-	 *
-	 * @param string $path
-	 * @return string
-	 */
 	public function getETag($path) {
 		return $this->calculateEtag($path, $this->stat($path));
 	}
 
-	private function calculateEtag(string $path, array $stat): string {
+	private function calculateEtag(string $path, array $stat): string|false {
 		if ($stat['mode'] & 0x4000 && !($stat['mode'] & 0x8000)) { // is_dir & not socket
 			return parent::getETag($path);
 		} else {