Make sure we have at least two timestamps in lastmodified calculation

Otherwise our max(*timestamps) call might fail

src/octoprint/server/views.py

@@ -530,7 +530,7 @@ def _files_for_index():
 
 def _compute_date(files):
 	from datetime import datetime
-	timestamps = map(lambda path: os.stat(path).st_mtime, files)
+	timestamps = map(lambda path: os.stat(path).st_mtime, files) + [0] if files else []
 	max_timestamp = max(*timestamps) if timestamps else None
 	if max_timestamp:
 		# we set the micros to 0 since microseconds are not speced for HTTP