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

从此 issue 中的 subprocess.Popen 示例开始，在 CPython 3.13/Linux 上比较 text=True 和 text=False 的非阻塞读取结果。阅读 issue #57531 了解提议的通用行为；当 no-data-yet 和 EOF 可以区分，且二进制流和文本流的行为一致时，即表示完成。