python: add default format for !d conversion specifier

Problem: The `!d` conversion specifier returns an empty string if
no format is provided, which could cause confusion in casual usage.
For example `{expiration!d}` returns an empty string even if there
is an expiration timestamp for a job.

If no format is provided, default to an ISO 8601 format "%FT%T".

Author: grondo

src/bindings/python/flux/util.py

@@ -318,7 +318,7 @@ def __format__(self, fmt):
             result = ""
         else:
             # Call strftime() to get the formatted datetime as a string
-            result = self.strftime(timefmt)
+            result = self.strftime(timefmt or "%FT%T")
 
         spec = spec[0] if spec else ""