In #57666 we changed when null_value was parsed for ip and date fields. Previously,
the null value was stored as a string, and parsed into a date or InetAddress whenever
a document containing a null value was encountered. Now, the values are parsed when
the mappings are built, which means that bad values are detected up front; if you try and
add a mapping with a badly-parsed ip or date for a null_value, the mapping will be
rejected.
This causes problems for upgrades in the case when you have a badly-formed null_value
in a pre-7.9 cluster. This commit fixes the upgrade case by changing the logic to only
logging a warning on the badly formed value, replicating the earlier behaviour.
Fixes#62363
91e2330529