[Bug fix]

In current version of log(), when you pass a zero size array, it will throw an error:
`ValueError: Unknown format code 'f' for object of type 'str'`.
This PR fixes it.

mrcnn/model.py

@@ -41,11 +41,12 @@ def log(text, array=None):
     """
     if array is not None:
         text = text.ljust(25)
-        text += ("shape: {:20}  min: {:10.5f}  max: {:10.5f}  {}".format(
-            str(array.shape),
-            array.min() if array.size else "",
-            array.max() if array.size else "",
-            array.dtype))
+        text += ("shape: {:20}  ".format(str(array.shape)))
+        if array.size:
+            text += ("min: {:10.5f}  max: {:10.5f}".format(array.min(),array.max()))
+        else:
+            text += ("min: {:10}  max: {:10}".format("",""))
+        text += "  {}".format(array.dtype)
     print(text)