Graphs have empty paths when bandwidth data is unavailable

For some relays Onionoo does not have bandwidth data for all time periods. For instance B743BE28571D87AEFF9C409756644DB5C64A7C45 is missing its 5 years data.

When data is unavailable the graphs are empty except for the legend. However the path elements are still created leading to W3C validator errors such as

Element path is missing required attribute d.

FWICT this issue was partially addressed by #19553 (moved) but was solved by removing more recent graphs instead of having Atlas detect and handle unavailable data.