Fixed bug in assign_upslope_vals_to_active_links.
Fixed bug in assign_upslope_vals_to_active_links.

Fixed bug in assign_upslope_vals_to_active_links where the default value
for *v* was causing a TypeError. Check if *v* is `None` and, if so,
assign it to an array of zeros of length one.

Carralex/landlab,amandersillinois/landlab,ManuSchmi88/landlab,RondaStrauch/landlab,landlab/landlab,Carralex/landlab,amandersillinois/landlab,decvalts/landlab,cmshobe/landlab,landlab/landlab,decvalts/landlab,ManuSchmi88/landlab,cmshobe/landlab,cmshobe/landlab,RondaStrauch/landlab,SiccarPoint/landlab,csherwood-usgs/landlab,csherwood-usgs/landlab,RondaStrauch/landlab,SiccarPoint/landlab,laijingtao/landlab,ManuSchmi88/landlab,laijingtao/landlab,landlab/landlab,Carralex/landlab