python: format all zero UtilDatetime objects as an empty string

Problem: It is inconvenient that UtilDatetime objects (obtained via the
`!d` conversion specifier) are treated differently than the normal
datetime conversion (`!D`) when the timestamp is zero. In the first
case, the datetime is rendered to the epoch date in whatever format
is specified by the user, while in the second case the result is an
empty string. Since a zero timestamp is considered "unset", the empty
string is much more preferable here.

Since the prime use case for UtilDatetime is in output format rendering
for utilities, have this class return an empty string by default for
zero timestamps, presuming these are unset.

Fix one test in t2800-jobs-cmd.t that assumed a 00:00 result.

Author: grondo

src/bindings/python/flux/util.py

@@ -314,8 +314,11 @@ def __format__(self, fmt):
         # any Python format spec:
         vals = fmt.split("::", 1)
 
-        # Call strftime() to get the formatted datetime as a string
-        result = self.strftime(vals[0])
+        if self == datetime.fromtimestamp(0.0):
+            result = ""
+        else:
+            # Call strftime() to get the formatted datetime as a string
+            result = self.strftime(vals[0])
 
         # If there was a format spec, apply it here:
         try:

t/t2800-jobs-cmd.t

@@ -1008,7 +1008,7 @@ test_expect_success 'flux-jobs emits empty string for special case t_estimate' '
 	flux jobs -no "${fmt}" >t_estimate_annotations.out 2>&1 &&
 	test_debug "cat t_estimate_annotations.out" &&
 	for i in `seq 1 $(state_count active)`; do
-		echo ",00:00,,,,-,-,-" >> t_estimate_annotations.exp
+		echo ",,,,,-,-,-" >> t_estimate_annotations.exp
 	done &&
 	test_cmp t_estimate_annotations.out t_estimate_annotations.exp
 '