Handle empty import_namespaces like import_names in metadata round-trip

[sueun-dev]

import_names and import_namespaces (both added in Metadata 2.5 / PEP 794) share a processor and are validated the same way, but only import_names special-cases the empty-list value. import_namespaces does not, so on 26.3:

• Metadata.from_raw({..., "import_namespaces": []}) is accepted, but as_rfc822() drops the field entirely, and parsing the output back gives None instead of [].
• A bare Import-Namespace: header raises invalid or unparsed metadata on parse: the value [""] reaches _process_import_names, where "".isidentifier() is false. A bare Import-Name: parses to [].

Both special cases ([""] -> [] on parse, [] -> "" on write) were added for import_names in e83801f (#948), the commit that introduced both fields, and weren't mirrored to import_namespaces. This extends both branches to cover it and adds a round-trip test parametrized over the two fields.

Reproduced on 26.3:

>>> from packaging.metadata import Metadata, parse_email
>>> m = Metadata.from_raw({"metadata_version": "2.5", "name": "x", "version": "1", "import_namespaces": []}, validate=False)
>>> print(parse_email(m.as_rfc822().as_string())[0].get("import_namespaces"))
None
>>> # the same input with import_names gives []

[henryiii]

In PEP 794, Import-Name and Import-Namespaces are not symmetric.

The Import-Name and Import-Namespace fields are “multiple uses” fields. Each entry of both fields MUST be a valid import name or can be empty in the case of Import-Name.

Only Import-Name is allowed to be empty. This is to allow no import at all to be represented, but it must be represented as an empty Import-Name.