Cleanup null records in libraries

[aaccomazzi]

Apparently there are cases where records in a user's library can get a "null" value in their bibcode field. The theory is that these are deleted bibcodes which no longer exist in SOLR.

1. We should verify that these entries represent deleted bibcodes by checking backup versions of the database
2. We should verify what part of the code inserts these nulls and decide if some exception needs to be generated or warning logged
3. We should run periodic cronjobs that scan the database and remove these entries.

[aaccomazzi]

An example of one such libraries is: https://ui.adsabs.harvard.edu/#/public-libraries/2rdsLcoaQSyp3Prie-EQAw