Improper nadgrids error

[mbloch]

Shouldn't throw an error for this kind of definition:

# Popular Visualisation CRS / Mercator (deprecated)
<3785> +proj=merc +a=6378137 +b=6378137 +lat_ts=0.0 +lon_0=0.0 +x_0=0.0 +y_0=0 +k=1.0 +units=m +nadgrids=@null +wktext +no_defs <>

[mbloch]

Fixed in af30a8668bf548c627