kpi: Fix Σqci and Σcause when invoked on NA data

There was a thinko that led to returning 0 instead of NA when there are
not .QCI or .CAUSE fields except .sum . Without added fix, e.g.

	Σqci(Measurement(), 'ERAB.EstabInitAttNbr.QCI')

was returning 0 instead of NA.

-> Fix it.

kpi.py

@@ -602,7 +602,7 @@ def _Σx(m: Measurement, name_x: str, _all_x: func):
     if not isNA(s):
         return s
     s  = s.dtype.type(0)
-    ok = True
+    ok = True  if len(name_xv) > 0  else False
     for _ in name_xv:
         v = m[_]
         # we don't know the answer even if single value is NA

kpi_test.py

@@ -18,7 +18,7 @@
 # See COPYING file for full licensing terms.
 # See https://www.nexedi.com/licensing for rationale and options.
 
-from xlte.kpi import Calc, MeasurementLog, Measurement, Interval, NA, isNA
+from xlte.kpi import Calc, MeasurementLog, Measurement, Interval, NA, isNA, Σqci, Σcause
 import numpy as np
 from pytest import raises
 
@@ -420,6 +420,34 @@ def test_Calc_erab_accessibility():
     _(InititialEPSBEstabSR, 100 * 2*3*4 / (7*8*9))
 
 
+# verify Σqci.
+def test_Σqci():
+    m = Measurement()
+    x = 'ERAB.EstabInitAttNbr'
+    def Σ():
+        return Σqci(m, x+'.QCI')
+
+    assert isNA(Σ())
+    m[x+'.sum'] = 123
+    assert Σ() == 123
+
+    # TODO sum over individual causes (when implemented)
+
+
+# verify Σcause.
+def test_Σcause():
+    m = Measurement()
+    x = 'RRC.ConnEstabAtt'
+    def Σ():
+        return Σcause(m, x+'.CAUSE')
+
+    assert isNA(Σ())
+    m[x+'.sum'] = 123
+    assert Σ() == 123
+
+    # TODO sum over individual causes (when implemented)
+
+
 def test_NA():
     def _(typ):
         na = NA(typ(0).dtype)