Don't write reaction targets that are None to XML

This is related to a bug pointed out in the user's group, where
writing a ReactionTuple with a None target errors.
https://groups.google.com/forum/#!topic/openmc-users/jB_-zEKhF-k
The condition appeared when the reaction was a non-fission
(n, gamma), but the daughter isotope did not exist in the Chain,
represented by None.

This issue is resolved by skipping the target only if daughter
is None, regardless of reaction. The Nuclide.from_xml can handle
a missing target just as it handles a target of "Nothing"
[case-insensitive]

openmc/deplete/nuclide.py

@@ -243,7 +243,7 @@ class Nuclide:
             rx_elem = ET.SubElement(elem, 'reaction')
             rx_elem.set('type', rx)
             rx_elem.set('Q', str(Q))
-            if rx != 'fission' or daughter is not None:
+            if daughter is not None:
                 rx_elem.set('target', daughter)
             if br != 1.0:
                 rx_elem.set('branching_ratio', str(br))