Remove unnecessary warning in map_to_aro and document return of empty…

… list if ARO invalid in map_to_aro

argnorm/lib.py

@@ -50,6 +50,8 @@ def map_to_aro(gene, database):
 
     Returns:
         ARO[result] (pronto.term.Term): A pronto term with the ARO number of input gene. ARO number can be accessed using 'id' attribute and gene name can be accessed using 'name' attribute.
+
+        If ARO mapping is invalid or doesn't exist, an empty list [] is returned.
     """
 
     if database not in ['ncbi', 'deeparg', 'resfinder', 'sarg', 'megares', 'argannot']:
@@ -67,5 +69,4 @@ def map_to_aro(gene, database):
         if result in ARO:
             return ARO[result]
         else:
-            return []
-            raise Warning('The gene does not have an ARO mapping')
+            return []