update changelog

Author: rcannood

CHANGELOG.md

@@ -239,4 +239,7 @@
   and thus can't be directly overriden (new values given by get_donor_batch_map is int) (PR #119).
 
 * Update flowsom mapping similarity so we subset to just markers to correct, and lisi to remove control samples
-  and unlabelled cells (PR #119).
+  and unlabelled cells (PR #119).
+
+* Fix `emd` crashing on `pd.concat()` when every donor/sample combination and cell type falls
+  below the 50 cell threshold. Both the horizontal and vertical path now return NaN (PR #131).