zipapp.get_interpreter() raises UnicodeDecodeError on invalid shebang bytes

[Tony123-tech]

zipapp.get_interpreter() raises UnicodeDecodeError when the shebang
line contains bytes that aren't valid in shebang_encoding, instead of
returning None.

Reproducer:

import zipapp, tempfile, os
d = tempfile.mkdtemp()
p = os.path.join(d, "a.pyz")
with open(p, "wb") as f:
    f.write(b"#!\xff\xfe\xff\xfe\nx")
print(repr(zipapp.get_interpreter(p)))

Actual:
UnicodeDecodeError: 'utf-8' codec can't decode byte 0xff in position 0: invalid start byte

Expected:
None (treat undecodable shebang as "no interpreter")

The function already returns None for "no shebang", so returning
None for an undecodable shebang is consistent. The CLI renders
None as <none>.

Tested on: main.