dotenv get exits with code 1 for empty string values

Description

dotenv get KEY treats an empty string value as missing because the CLI uses a truthiness check (if stored_value:) instead of testing for key presence / None.

Empty values are valid in .env files (KEY= or KEY=""). The library API already returns them correctly via get_key / dotenv_values; only the CLI get command is wrong.

Steps to reproduce

import subprocess, sys, tempfile
from pathlib import Path
from dotenv import set_key

with tempfile.TemporaryDirectory() as directory:
    path = Path(directory) / ".env"
    set_key(path, "EMPTY", "")
    result = subprocess.run(
        [sys.executable, "-m", "dotenv", "-f", str(path), "get", "EMPTY"],
        capture_output=True,
        text=True,
    )
    print(result.returncode, repr(result.stdout))

Expected behavior

0 ''

(exit code 0, empty value printed)

Actual behavior

1 ''

(exit code 1 — same as a missing key)

dotenv get ZERO with ZERO=0 works (exit 0), so only falsy-but-present empty strings are affected.

Environment

• python-dotenv: 1.2.3 (current main)
• Python: 3.14
• OS: Windows

Suggested fix

In src/dotenv/cli.py get, distinguish missing/None from an empty string, e.g. exit only when the key is absent or the stored value is None.

Start in src/dotenv/cli.py at the dotenv get command and inspect how it distinguishes a missing key from an empty value. Reproduce the issue with the Python subprocess example in the report; done means an existing empty value prints an empty string and exits with code 0, while a missing key still exits with code 1.