Fix node import for nodes with null install dates (#392)

src/meshdb/utils/spreadsheet_import/parse_node.py

@@ -1,3 +1,4 @@
+import datetime
 import logging
 from typing import Optional
 
@@ -111,7 +112,7 @@ def normalize_building_node_links(building: models.Building, node: models.Node):
         node
         for node in sorted(
             active_nodes if active_nodes else nodes_in_cluster,
-            key=lambda node: node.install_date,
+            key=lambda node: node.install_date or datetime.datetime.max,
         )
     ][0]