In a Google+ post, Google announced that they will show null Quality Scores in AdWords for the keywords that are new and when the Quality Score can not be determined due to lack of enough clicks or impressions.
Google said, “To provide more transparency, null Quality Scores will appear for new keywords, as well as keywords that lack recent click and impression data. Null Quality Scores will be excluded if you use reports or automated rules that include a filter on the Quality Score column. To show null scores, check the new "Include keywords with quality scores of ‘–’ ” option when you set up your rule or filter.”
Google suggested reviewing the reports, filters, rules and scripts that rely on Quality Scores before September 12, 2016.Google To Give Null Quality Scores For New Keywords And Keyword With Low Activity!,