Fix a bunch of typing issues to make PHPStan level 10 happy on groupfolders

apps/dav/lib/Connector/Sabre/File.php

@@ -236,7 +236,13 @@ public function put($data) {
 					// because we have no clue about the cause we can only throw back a 500/Internal Server Error
 					throw new Exception($this->l10n->t('Could not write file contents'));
 				}
-				[$count, $result] = Files::streamCopy($data, $target, true);
+				$count = stream_copy_to_stream($data, $target);
+				if ($count === false) {
+					$result = false;
+					$count = 0;
+				} else {
+					$result = true;
+				}
 				fclose($target);
 			}
 			if ($result === false && $expected !== null) {

apps/dav/lib/Connector/Sabre/FilesPlugin.php

@@ -166,11 +166,6 @@ public function checkCopy($source, $target): void {
 			return;
 		}
 
-		// Ensure source exists
-		$sourceNodeFileInfo = $sourceNode->getFileInfo();
-		if ($sourceNodeFileInfo === null) {
-			throw new NotFound($source . ' does not exist');
-		}
 		// Ensure the target name is valid
 		try {
 			[$targetPath, $targetName] = \Sabre\Uri\split($target);