Fix dotenv get exiting with code 1 for empty string values

[ShamikOfficial]

Fixes #699.

Problem

dotenv get KEY uses if stored_value: so an empty string is treated as missing and the command exits with code 1 — the same as an absent key.

>>> from dotenv import set_key, get_key
>>> set_key(".env", "EMPTY", "")
>>> get_key(".env", "EMPTY")   # library API is fine
''
>>> # dotenv get EMPTY  -> exit code 1

Empty values (KEY= / KEY="") are valid; only the CLI was wrong.

Fix

Exit only when the key is absent or the stored value is None (bare key with no value). Empty strings are echoed and exit 0.

Tests

• New test_get_empty_string_value pins exit code 0 and a blank line of output for a=.
• Existing get tests still pass.

pytest tests/test_cli.py::test_get_existing_value tests/test_cli.py::test_get_empty_string_value tests/test_cli.py::test_get_non_existent_value — 3 passed.