fix: do not treat an inline comment as an empty value

[dchaudhari7177]

Fixes #600.

Problem

An inline comment after an empty unquoted value becomes the value:

>>> dotenv_values(stream=io.StringIO("WITH_VALUE=hello  # c\nEMPTY_VALUE=  # c\n"))
OrderedDict({'WITH_VALUE': 'hello', 'EMPTY_VALUE': '# c'})

EMPTY_VALUE should be '', the same way WITH_VALUE gets its comment stripped.

Cause

_equal_sign is (=[^\S\r\n]*), so it consumes the horizontal whitespace right after =. parse_unquoted_value then strips comments with re.sub(r"\s+#.*", "", part), which requires whitespace before the #. For KEY= # c that whitespace is already gone, the part is # c with the # at position 0, the substitution does not fire, and the comment is returned as the value.

Fix

parse_binding already gets the matched = plus whitespace back from read_regex, so it can tell whether any whitespace followed the =. That flag is threaded to parse_unquoted_value, which treats a part starting with # as a comment only when the whitespace was there.

This keeps the existing rule that # starts a comment only when preceded by whitespace, so a=b#c and the newly-covered a=#c both still parse to a literal value containing the #.

Input	Before	After
a=b # c	b	b
a= # c	# c	`` (empty)
a=#c	#c	#c
a=b#c	b#c	b#c
a=	`` (empty)	`` (empty)

Tests

Four cases added to test_parse_stream, including the no-whitespace a=#c case that pins the unchanged behaviour and a two-binding case confirming the reader is left in the right position.

pytest tests/ passes. The two test_cli.py::test_run_* failures on my machine are pre-existing and Windows-specific (they reproduce on an unmodified checkout).

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