fix(migration): don't reclassify custom ssl sites as letsencrypt

[mrrobot47]

Problem

The 20250927101545_..._fix_ssl_flag_for_existing_le_certs migration auto-sets site_ssl = 'le' for sites that have LE-looking cert files on disk and a Let's Encrypt issuer. Its gate empty( $db_ssl ) || $db_ssl !== 'le' matches any non-le site — including ones explicitly created with --ssl=custom (also self/inherit). A custom site using a Let's Encrypt-issued cert (with a .chain.pem present) would be silently reclassified to le and pulled into automatic LE renewal that can overwrite or break the user's certificate.

Fix

Narrow the gate to empty( $db_ssl ) — repair only sites whose SSL flag is genuinely unset (the migration's intent). Explicit custom/self/inherit (and le) choices are never overridden. down() remains a no-op (forward-only). The boolean change preserves the un-flagged-LE repair case and removes only the unwanted override (the old || $db_ssl !== 'le' half was already redundant for the le case).

Note

A site whose flag was explicitly set to a wrong non-empty value will no longer be auto-corrected — an intentional, conservative trade-off: a data migration should not override an explicit user choice.

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