Payload null value exception handling [LUCENE-8424]

If-Condition to check payload null value in _PayloadScoreQuery.java_ was removed in #9086. Because of that, regarding of the payload value, _payloadsSeen_ is always getting increment. This has compromised the overall score of the document(for instance, as payloadSeens is always greater than 0, docScore() in _MinPayloadFunction.java_ only returns payloadScore which can be zero depending on the payload value). Ideally, it should've returned 1 in case of _payload==null_.

I have added a simple patch to get started, Please let me know if it needs any improvements. 
Thanks in advance.

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Migrated from [LUCENE-8424](https://issues.apache.org/jira/browse/LUCENE-8424) by Tapan Vaishnav, updated Nov 01 2018
Attachments: [LUCENE-8424.patch](https://apache.github.io/lucene-jira-archive/attachments/LUCENE-8424/LUCENE-8424.patch) (versions: 7)