GeneGC, GeneGCBatch update to account for ncRNAs with GC 0

C. elegans has 17 ncRNAs that have GC content 0, therefore would not get an attrib_type GeneGC.
The proposed changes are to allow GeneGC attributes to be added with value = 0.
example:
http://metazoa.ensembl.org/Caenorhabditis_elegans/Gene/Summary?db=core;g=WBGene00197932;r=II:2969192-2969248;t=K08A2.13