update cast_timestamp_to_iso8601 to work with timestamp and string (#…

…1473)

src/ol_dbt/macros/cast_timestamp_to_iso8601.sql

@@ -1,3 +1,9 @@
 {% macro cast_timestamp_to_iso8601(column_name) %}
-    to_iso8601(from_iso8601_timestamp({{ column_name }}))
+  case
+       -- If the column is already a timestamp, convert to ISO 8601
+        when try_cast({{ column_name }} AS timestamp) is not null
+           then to_iso8601(try_cast({{ column_name }} AS timestamp))
+        -- otherwise, convert to ISO 8601 from a string
+        else to_iso8601(from_iso8601_timestamp(try_cast({{ column_name }} AS varchar)))
+   end
 {% endmacro %}