Fix datashader 0.15.0 single category bug

Author: Sean Sullivan

elastic_datashader/tilegen.py

@@ -1271,6 +1271,10 @@ def remap_bucket(bucket, search):
                 categories.insert(0, categories.pop(categories.index("Other")))
             except ValueError:
                 pass
+            
+            # datashader has issue in 0.15.0 with categories that have only one class
+            if len(categories) == 1:
+                categories.append('')
             cat_dtype = pd.api.types.CategoricalDtype(categories=categories, ordered=True)
             df["t"] = df["t"].astype(cat_dtype)