Changes Coming to Quality Score reporting
Starting the week of September 12 , 2016, you’ll start seeing null Quality Scores (designated by "--" in the table) when there aren’t enough impressions or clicks to accurately determine a keyword’s Quality Score.
A result of "--" will appear for new keywords, as well as keywords that lack recent click and impression data. By default, Quality Scores of "--" won’t be included when filtering by Quality Score in your reports or automated rules. But you'll have the option of including Quality Scores of “--” in your filtered data.
Read More Here: https://support.google.com/adwords/answer/7050591?hl=en