Fix assumption of existing collection
Fix assumption of existing collection

* When logging in, if a user selects "Import existing collections", the
  system would throw an exception, since Redis would return a NoneType,
  which would then be accessed by json.loads(), which doesn't seem to
  know how to process that.
  This fixes that, by providing a string representing an empty json
  array ("[]"), in place of the NoneType.

DannyArends/genenetwork2,genenetwork/genenetwork2,DannyArends/genenetwork2,DannyArends/genenetwork2,genenetwork/genenetwork2,zsloan/genenetwork2,genenetwork/genenetwork2,pjotrp/genenetwork2,genenetwork/genenetwork2,pjotrp/genenetwork2,DannyArends/genenetwork2,DannyArends/genenetwork2,pjotrp/genenetwork2,DannyArends/genenetwork2,zsloan/genenetwork2,zsloan/genenetwork2,pjotrp/genenetwork2,pjotrp/genenetwork2,zsloan/genenetwork2