Pass encoding='utf-8', errors='ignore' and guard against result.stdout
being None so _scan_gateway_pids() no longer crashes with
UnicodeDecodeError + AttributeError on Windows systems whose default
code page is not UTF-8 (e.g. cp936 on zh-CN). The parser only matches
the ASCII prefixes CommandLine= and ProcessId=, so dropping undecodable
bytes is safe.
Closes#17049.
