We should support null to indicate that the value is not available. We should also support notNull field schema parameter to indicate that the value must exist.
W3C version of the tabular schema also supports "json-y" way of values - instead of storing data as a list of lists [ [1,2,3], [4,5,6] ], it also allows an object approach - [ {"hdr1": 1, "hdr2": 2, "hdr3": 3}, { ... } ]. This allows for an easy way to indicate missing values - simply don't specify it. It is almost the same as setting it to null, but not entirely the same: missing != null. If we don't support it, we basically equate null and missing.