Improved testing of expressions with empty input

[jlowe]

#11183 is another case of an issue with properly handling empty input. There needs to be better testing of the empty input corner case for GPU expressions. Note that "empty" here refers to many different things, all of which can be considered "empty" in one way or another:

• batch with zero rows
• string column with only null or empty strings (i.e.: data column is empty)
• column with all nulls (i.e.: empty in the sense that there are no valid data values)