For empty assembly, zero length is an incorrect attribute

Scenario

1. Select "Tools->NGS data analysis->Extract consensus from assemblies..."
2. Set "_common_data/ugenedb/1.bam.ugenedb" as an input
3. Click "Run"
   Actual state: there are 168 messages in the log with text "ignored incorrect value of attribute reference_length_attribute: should be > 0, got 0. Bad attribute removed!"
   Expected state: no messages

1.bam.ugenedb contains 168 empty assemblies. Each assembly has a reference_length_attribute of 0, which is considered incorrect. I suggest making the value 0 correct for reference_length_attribute, if assembly is empty