insights: insights return no data points for 0 values

Created by: leonore

loom: https://www.loom.com/share/463efcf0ba6a474d863cf6d64701af45

this doesn't happen in the live preview- so there's a discrepancy in behaviour

when there are no historical captured value for a result (e.g. https://sourcegraph.sourcegraph.com/search?q=context:global+file:go%5C.mod%24+go%5Cs*%28%5Cd%5C.%5Cd%2B%29+%28.%5Cn*%29%2B+github.com/sourcegraph/log&patternType=regexp we have no captured results beyond a certain time), we return null

/cc @joelkw @felixfbecker @vovakulikov @unclejustin