minor code cleanup

Author: CeresBarros

Biomass_regenerationPM.R

@@ -60,7 +60,7 @@ defineModule(sim, list(
     expectsInput("studyArea", "SpatialPolygonsDataFrame",
                  desc = paste("Polygon to use as the study area.",
                               "Defaults to  an area in Southwestern Alberta, Canada."),
-                 sourceURL = ""),
+                 sourceURL = NA),
     expectsInput("sufficientLight", "data.frame",
                  desc = "table defining how the species with different shade tolerance respond to stand shadeness",
                  sourceURL = "https://raw.githubusercontent.com/LANDIS-II-Foundation/Extensions-Succession/master/biomass-succession-archive/trunk/tests/v6.0-2.0/biomass-succession_test.txt"),