Constraining to null values results in a 400 server error.

[yochannah]

Steps to reproduce using flymine default public lists

1. select PL FlyTF_site_specific_TFs from the list page
2. Go to the column summary icon (bar chart) for symbol, and select the NO VALUE option, then click filter.
3. Error. Error. Computer explodes and 400 is returned.

This might be imtables, not imcljs.

[yochannah]

[ERROR] 400 XML is not well formatted. Got <query model="genomic" view="Gene.id">
   <constraint path="Gene" value="PL FlyTF_site_specific_TFs" op="IN" code="A"/>
   <constraint path="Gene.symbol" op="ONE OF" code="B"><value></value></constraint>
</query>.