I don't know how likely this is in practice, though just in case, reporting it as a security issue anyway:
hash_pbkdf2() can return false, for example if the specified algorithm is unknown, which allows a simple misconfiguration, like this:
$wgPasswordConfig['pbkdf2']['algo'] = 'sha-512'; // note the hyphen
to cause any newly generated password hash (e.g. :pbkdf2:sha-512:10000:128:jyK0+sEmJ+CMnpucRLANmg==:) to match any password. The same could also happen when comparing against a password hash generated using an algorithm supported only in a newer version of PHP.
hash_pbkdf2() generates a PHP warning; however, if display_errors is off (as it should be in production), they would just go into a log file that the server administrator may or may not check. The function then returns false; however, base64_encode() treats its false return value as the empty string.
It would be entirely reasonable to add an is_string() check to prevent this. In fact, BcryptPassword already has one. It may also make sense to add sanity checks elsewhere, such as in toString() and LayeredParameterizedPassword, to protect against similar issues.