non-blocking reads of binary and text streams handle no-data differently

Bug report

Bug description:

proc = subprocess.Popen(
    ['bash', '-c', 'sleep 10; echo hello'],
    stdout=subprocess.PIPE,
    text=True
)
os.set_blocking(proc.stdout.fileno(), False)
print(repr(proc.stdout.read(8192)))

Refer to issue #57531 for discussion on what should happen in general, but I'm filing this to point out that when text=True the output is an empty string (indistinguishable from EOF, which is a problem), and when text=False the result is None (while an empty string is produced only on EOF).

They should probably behave consistently, and also EOF should be distinguishable from no-data-yet; making the binary behaviour preferable.

CPython versions tested on:

3.13

Operating systems tested on:

Linux

從此 issue 中的 subprocess.Popen 範例開始，在 CPython 3.13/Linux 上比較 text=True 和 text=False 的非阻塞讀取結果。閱讀 issue #57531 了解提議的一般行為；當 no-data-yet 和 EOF 可以區分，且二進位串流和文字串流的行為一致時，即表示完成。