[#347] remove stderr check

Signed-off-by: m.malygina <m.malygina@yadro.com>

src/frostfs_testlib/processes/remote_process.py

@@ -193,7 +193,7 @@ class RemoteProcess:
         )
         if "No such file or directory" in terminal.stderr:
             return None
-        elif terminal.stderr or terminal.return_code != 0:
+        elif terminal.return_code != 0:
             raise AssertionError(f"cat process {file} was not successful: {terminal.stderr}")
 
         return terminal.stdout