Summary:
Previously, cloudwatch data sources would only fill in a single null value if
there was missing data. This results in behavior described in #9267. This
resolves that issue by filling the entire missing period with null values. The
null values can then be interpreted as normal by the graphing frontend.

Test Plan:
Used on a data source that had missing data for many consecutive periods.
Ensured that the graph remained at 0 across the entire window.