Fix unit tests

Author: vlmed

lib/internal/Magento/Framework/File/Mime.php

@@ -115,7 +115,7 @@ public function getMimeType($file)
 
         $mimeType = '';
         if ($driver instanceof Filesystem\DriverInterface) {
-            $mimeType = $driver->getMetadata($file)['mimetype'];
+            $mimeType = $driver->getMetadata($file)['mimetype'] ?? '';
         }
 
         if (!$mimeType) {

lib/internal/Magento/Framework/File/Test/Unit/MimeTest.php

@@ -83,7 +83,8 @@ public function testGetMimeTypeNonexistentFileException(): void
             ->method('isExists')
             ->with('nonexistent.file')
             ->willReturn(true);
-
+        $this->localDriverMock->method('getMetadata')
+            ->willThrowException(new FileSystemException(__('File \'nonexistent.file\' doesn\'t exist')));
         $file = 'nonexistent.file';
         $this->object->getMimeType($file);
     }
@@ -103,7 +104,8 @@ public function testGetMimeType($file, $expectedType): void
             ->method('isExists')
             ->with($file)
             ->willReturn(true);
-
+        $this->localDriverMock->method('getMetadata')
+            ->willReturn(['mimetype' => $expectedType]);
         $actualType = $this->object->getMimeType($file);
         self::assertSame($expectedType, $actualType);
     }