Fixing the elasticnet prediction functions work with cross-validation

When elasticnet net selects no variables at all during cross-validation (high penalty), the prediction functions generate empty arrays because they only use the selected variables. These empty arrays (array[]) then cause SQL syntax errors. This patch fixes the problem.

Jira: MADLIB-824
Signed-off-by: Ronert Obst <ronert.obst@gmail.com>