Describe the bug
PR: #33

• Save a saved search in discover that uses is one of filter. If only one item is added in the filter, server crashes:

server    log   [18:37:58.412] [error][opendistroReportsKibana][plugins] Failed to generate report: Error: Query string is required for full text query!
server    log   [18:37:58.413] [error][opendistroReportsKibana][plugins] Error: Query string is required for full text query!
    at MatchPhraseQuery.toJSON (/Users/lijshu/Projects/7.10.2/kibana/.kibana-reports/kibana-reports/node_modules/elastic-builder/lib/queries/full-text-queries/mono-field-query-base.js:88:23)
...

• If there are multiple items in a negated is one of filter, csv report is inconsistent with discover results since filters are not handled correctly

To Reproduce

Expected behavior
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• OS: [e.g. iOS]
• Version [e.g. 22]

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