flatten in cases where an empty nested array would confuse join.

Since I offered the original PRs to support modules in scald.rb, local use has revealed an issue.
Basically, stray commas (fatal) and colons (not-fatal, but bad form) were appearing in libjar/classpaths whenever no extra scalding modules were specified.  In particular, this hadoop_command:

HADOOP_CLASSPATH=/usr/share/java/hadoop-lzo-0.4.15.jar:scalding-core-assembly-0.9.0rc4.jar::job-jars/t0.jar hadoop jar scalding-core-assembly-0.9.0rc4.jar -libjars ,job-jars/t0.jar -Dmapred.reduce.tasks=20 -Dmapred.min.split.size=2000000000 t0 --hdfs --input zero --output b

appeared to hang upon execution.  I am sorry that I sent the PR after only testing the more complex --hdfs cases with extra modules but failed to retest all the simple cases.