Fix error for geom with all missing data

[tdhock]

Currently the error we get for the new test case is

> animint2dir(viz)
Error in split.data.table(x, by = vars, keep.by = FALSE, drop = TRUE) (from z_animintHelpers.R#879) : 
  Argument 'by' must refer to column names in x

this error could be changed so that it helps the user fix the issue in their code.

[tdhock]

No obvious timing issues in HEAD=fix-missing-err

Generated via commit 2a4a35b

Download link for the artifact containing the test results: ↓ atime-results.zip

Task	Duration
R setup and installing dependencies	3 minutes and 3 seconds
Installing different package versions	1 minutes and 0 seconds
Running and plotting the test cases	2 minutes and 19 seconds