show a warning for missing/invalid values in channels.

[Fil]

For discussion, as a way to address #493 by adding a warning (#755) when some channel values are ignored.

Q: should this be opt-in or opt-out?

[Fil]

word-cloud.js is an example where we deliberately use an empty value (the text mark filters out "") to choose the words we display:

text: (d) => (d.length > 1 ? `${d[0]} (${d.length})` : ""),

Having to opt-out by setting ignoreMissing: true in this case would be unfortunate. Maybe we need two pass filters: first, for missing/invalid values (only checking undefined and NaN), and warning about it; second, the mark's normal filter. However this doesn't really work for the case of the dot mark which rejects negative values.