ALMaSS: fix logic when choosing low or high nutrient growth curves

Previously, we chose the lownutrientgrowth curve when the was a
`fertiliser` event for the cropstate, otherwise the highnutrientgrowth
curve.

It should be the other way around.

src/crop/almass.jl

@@ -599,8 +599,17 @@ end
 get_dddegs(model::SimulationModel) = bounds(supply_temperature(model))
 
 # TODO efficiency: the fertiliser check is done in many places
-growthcurve(cs::CropState) =
-    fertiliser in cs.events ? cs.croptype.lownutrientgrowth : cs.croptype.highnutrientgrowth
+function growthcurve(cs::CropState)
+    curve = if fertiliser in cs.events
+        cs.croptype.highnutrientgrowth
+    else
+        cs.croptype.lownutrientgrowth
+    end
+    if ismissing(curve)
+        error("Growth curve is missing for cropstate:\n $cs")
+    end
+    return curve
+end
 
 # Function in original ALMaSS code:
 #     `double PlantGrowthData::GetStartValue(int a_veg_type, int a_phase, int a_type)` in `Landscape/Plants.h`, line 142