use specified costs when one of the strings is empty

If one of the strings is empty it's not correct to assume that the cost is the length of the other string since we the insertion and deletion costs are configurable and not necessarily 1.

strsimpy/weighted_levenshtein.py

@@ -18,6 +18,7 @@
 # OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
 # SOFTWARE.
 
+from functools import reduce
 from .string_distance import StringDistance
 
 
@@ -52,9 +53,9 @@ class WeightedLevenshtein(StringDistance):
         if s0 == s1:
             return 0.0
         if len(s0) == 0:
-            return len(s1)
+            return reduce(lambda cost, char: cost + self.insertion_cost_fn(char), s1, 0)
         if len(s1) == 0:
-            return len(s0)
+            return reduce(lambda cost, char: cost + self.deletion_cost_fn(char), s0, 0)
 
         v0, v1 = [0.0] * (len(s1) + 1), [0.0] * (len(s1) + 1)