correct exposures with total zero value

Author: gabrielaznar

climada/entity/exposures/black_marble.py

@@ -506,4 +506,7 @@ def _set_econ_indicators(exp, gdp, inc_grp):
         inc_grp (float): index to weight exposures in the region
     """
     exp.value = np.power(exp.value, 3)
-    exp.value = exp.value/exp.value.sum()* gdp * (inc_grp+1)
+    if exp.value.sum() > 0:
+        exp.value = exp.value/exp.value.sum()* gdp * (inc_grp+1)
+    else:
+        exp.value = 0.0