Fix empty drop creation when empty trans-units are pending translation

[wadimw]

Scenario

An empty trans-unit gets pushed to Mojito.

<trans-unit id="" resname="100_character_description_" datatype="php">
    <source/>
</trans-unit>

It shows up with Needs Translation status but has 0 words.

A drop export is triggered for the repository where all other trans-units have already been translated.

Drop will still get created, because Mojito sees that some trans-unit still needs translation - however, this drop will be empty because a 0-word trans-unit is not included in the drop.

Fix

Updated drop-export command to check if any words need translation, rather than trans-units.

[wadimw]

This is probably the simplest solution for this case of empty drop creation problem, though it's not complete.

One could argue that:
a. an empty trans-unit should never have a Needs Translation status
b. an empty trans-unit should not be checked in to Mojito at all
c. an empty trans-unit with Needs Translation status should in fact be exported in a drop; a drop import would then accept an empty <target/> (just like a push accepts the empty <source/>) and just carry on as usual (i.e. advance trans-unit status to Pending Review)

Additionally, it's worth noting that there is currently no way to manually change status of an empty trans-unit in Webapp UI, so if we decide on c. we should fix that as well.