Google Adwords has delayed its Null quality score reporting change for one month. It was supposed to be updated on 12th September 2016. Now the new date is October 10, 2016. Advertisers on Google Adwords will see null (designated “--” in the table) from 10th October 2016 for the keywords that don’t have enough impressions or clicks to accurately determine a keyword’s quality score.
“--” will also appear for the newly added keywords. By default, Quality scores of "--" won’t be included when filtering by Quality Score in your reports or automated rules.
Source: Adwords Support