When indexing a non-required and empty integer attribute, an error occures. Solr dump:

SEVERE: org.apache.solr.common.SolrException: Error while creating field 'attr_width_of_parcel_si

{type=sint,properties=indexed,stored,omitNorms,multiValued,sortMissingLast}

' from value ''
...
Caused by: java.lang.NumberFormatException: For input string: ""
...

Steps to reproduce

Create a class with non-required integer attribute
Create a new object of that class, empty the attribute (clear out default value)
When indexing new object, an error occures and the object is not inserted to index.