[Checkins] [zopefoundation/Products.ZCatalog] bdd820: Add IIndexingMissingValue support

[Andreas Gabriel]

  Branch: refs/heads/not-parm-patch
  Home:   https://github.com/zopefoundation/Products.ZCatalog
  Commit: bdd8206511608181dc395fa7d77e723d41d45895
      https://github.com/zopefoundation/Products.ZCatalog/commit/bdd8206511608181dc395fa7d77e723d41d45895
  Author: Andreas Gabriel <gabriel at hrz.uni-marburg.de>
  Date:   2019-04-15 (Mon, 15 Apr 2019)

  Changed paths:
    M src/Products/PluginIndexes/KeywordIndex/KeywordIndex.py
    M src/Products/PluginIndexes/KeywordIndex/tests.py
    M src/Products/PluginIndexes/interfaces.py
    M src/Products/PluginIndexes/unindex.py

  Log Message:
  -----------
  Add IIndexingMissingValue support