fix(task/backend): return useful error when validating empty script
fix(task/backend): return useful error when validating empty script

After we started parsing options inside of the task validation methods,
we accidentally shadowed the "missing script" error when the script was
empty. This restores the behavior of telling the user "missing script"
rather than a cryptic syntax error.

influxdata/influxdb,nooproblem/influxdb,mark-rushakoff/influxdb,li-ang/influxdb,influxdb/influxdb,mark-rushakoff/influxdb,mark-rushakoff/influxdb,nooproblem/influxdb,influxdata/influxdb,nooproblem/influxdb,influxdb/influxdb,influxdb/influxdb,influxdb/influxdb,influxdb/influxdb,mark-rushakoff/influxdb,influxdb/influxdb,nooproblem/influxdb,mark-rushakoff/influxdb,nooproblem/influxdb,li-ang/influxdb,mark-rushakoff/influxdb