pcp.pmi: fix pmiWrite when sec=0 and usec is omitted

The condition 'if sec and not usec' fails when sec==0 because 0 is
falsy, leaving usec as None and causing pmiWrite2(0, None) to raise
TypeError against its c_int argtypes.  Change to 'if usec is None'
so the datetime/float dispatch and the usec=0 fallback always run
when usec was not supplied by the caller.

Add a pmiWrite(0) call at the start of qa/src/test_pmi.python to
cover this path, and update qa/708.out accordingly.

Signed-off-by: Nathan Scott <nathans@redhat.com>
Co-Authored-By: Claude Sonnet 4.6 (1M context) <noreply@anthropic.com>

Author: natoscott

qa/708.out

@@ -5,6 +5,7 @@ pmiSetTimezone: UTC
 pmid 251.1.1
 units 
 pmiAddMetric: qa.one
+pmiPutValue: qa.one (epoch)
 pmiPutValue: qa.one
 pmid 251.1.2
 indom 251.3
@@ -26,7 +27,7 @@ Note: timezone set to local timezone of host "fu.bar.com" from archive
 qa.one
     Data Type: 32-bit unsigned int  InDom: PM_INDOM_NULL 0x........
     Semantics: discrete  Units: none
-    value 42
+    value 1
 
 qa.two
     Data Type: float  InDom: 251.3 0x........

qa/src/test_pmi.python

@@ -52,6 +52,12 @@ def test_pmi(self, path = OUTFILE, inherit = 0):
     print("pmiAddMetric: qa.one")
     self.assertTrue(code >= 0)
 
+    # exercise pmiWrite(0) - sec=0 with no usec previously raised TypeError
+    code = log.pmiPutValue("qa.one", "", "1")
+    print("pmiPutValue: qa.one (epoch)")
+    self.assertTrue(code >= 0)
+    log.pmiWrite(0)
+
     # give it a value
     code = log.pmiPutValue("qa.one", "", "42")
     print("pmiPutValue: qa.one")

src/python/pcp/pmi.py

@@ -386,7 +386,7 @@ def pmiWrite(self, sec: Union[int, float, datetime],
         status = LIBPCP_IMPORT.pmiUseContext(self._ctx)
         if status < 0:
             raise pmiErr(status)
-        if sec and not usec:
+        if usec is None:
             if isinstance(sec, datetime):
                 sec = float((sec - self._epoch).total_seconds())
             if isinstance(sec, float):