[5.x] Ensure null values are filtered out in dictionary field config

[duncanmcclean]

This pull request fixes an issue I spotted where null values weren't being filtered out when saving a dictionary field's config options.

-
  handle: dictionary_field
  field:
    dictionary:
      type: countries
      region: null
      emojis: true
    type: dictionary
    display: 'Dictionary Field'

^ In this example, the region key shouldn't be persisted when its null.