If the component field is not filled in, it will not show up in XML of the component after the component is saved. To verify the same, try saving the component and view the source. If you do not add any value, the source tab will not even have the field. Only once you add a whitespace or some value, the field starts to show up in the source tab.
Since the field is not even there in the component, Solr cannot search and find the same.
If you add a space in the field and save and then search using advanced search for schema and the field Name in Tridion and search by just hitting a space bar in search for field name, it will give the component with blank field.
I tried it in Tridion 2013 SP1 and it works fine and finds components where I had given a whitespace in the field.