Allow asn_lookup --update-database to create a missing database file

[cristianchiriac]

Fixes #2689.

Bug

ASNLookupExpertBot.update_database() rejects the database file whenever it doesn't already exist:

for database_path in set(bots.values()):
    if not Path(database_path).is_file():
        raise ValueError('Database file does not exist or is not a file.')
    elif not os.access(database_path, os.W_OK):
        raise ValueError('Database file is not writeable.')

But the actual write further down (pyasn.mrtx.dump_prefixes_to_file) creates the file fresh, and the parent directory is already created if needed (database_dir.mkdir(parents=True, exist_ok=True), right before the write). So this check blocks the documented first-time setup: as reported, the file has to be pre-created with "whatever content" before --update-database will even attempt to run.

Sibling expert bots with the same "download and cache a database" pattern (domain_suffix, domain_valid, maxmind_geoip, recordedfuture_iprisk, tor_nodes) don't have this pre-existence check at all — asn_lookup is the outlier.

Fix

Only reject a path that already exists and isn't a plain file, or an existing file that isn't writeable. A missing file is left alone — it's created by the update itself.

Test plan

• Added TestASNLookupUpdateDatabase with three cases: a missing database file is now allowed through (reaches the mocked network step instead of raising), an existing unwritable file is still rejected, and an existing directory at the database path is still rejected.
• Verified the core existence/writability check logic in isolation. I don't have a Linux/WSL environment set up here to run the full suite (intelmq.lib.utils imports grp and fcntl unconditionally, so the package doesn't import at all on native Windows) — CI should confirm the added tests pass.