fix: allow filters using false-y but not undefined values or ranges

[airhorns]

Before this change, the filter code was erroneously omitting filters that checked if a cell value was equal to 0, as 0 is not truth-y in JS. It is however a valid byte string to check against inside bigtable, so we should still send it along as a filter. This bug applied to both {value: 0} exact value filters, or {value: {start: 0}} style range filters, and this commit fixes both.