When using a DataProviderCompiler(), a blank column returned by the DataProvider causes an entire line to not be outputted in the result. I tracked this to a line in TemplateDataListener.newCell which explicitly filters out blank cells, even though such cells should be allowed.

The template I'm using is:
<snip>
template header
ruleID
object
field
regexp
replacement

package standardization

import [removed].*

template std_rule
rule "@

{ruleID}

"
dialect "mvel"
when
$o: @

{object}

()
then
$o.@

{field} = $o.?@{field}

.replaceFirst("@

{regexp}

", "@

{replacement}

");
end

end template
<snip>

Whenever "replacement" is blank, the consequent does not get generated.