Fix error calling MarshalJSON for type *influxdb.Result: json: unsupported value: NaN

If the MapMean did not have any values in the iterator, it would return
and empty meanMapOutput().  ReduceMean would in turn divider the results
by an unitialized float64 producing a NaN.  NaN is not serializable to JSON
by default.

influxql/functions.go

@@ -213,7 +213,12 @@ func MapMean(itr Iterator) interface{} {
 		out.Count++
 		out.Mean += (v.(float64) - out.Mean) / float64(out.Count)
 	}
-	return out
+
+	if out.Count > 0 {
+		return out
+	}
+
+	return nil
 }
 
 type meanMapOutput struct {

influxql/functions_test.go

@@ -0,0 +1,67 @@
+package influxql
+
+import "testing"
+
+type point struct {
+	seriesID  uint64
+	timestamp int64
+	value     interface{}
+}
+
+type testIterator struct {
+	values []point
+}
+
+func (t *testIterator) Next() (seriesID uint64, timestamp int64, value interface{}) {
+	if len(t.values) > 0 {
+		v := t.values[0]
+		t.values = t.values[1:]
+		return v.seriesID, v.timestamp, v.value
+	}
+	return 0, 0, nil
+}
+
+func TestMapMeanNoValues(t *testing.T) {
+	iter := &testIterator{}
+	if got := MapMean(iter); got != nil {
+		t.Errorf("output mismatch: exp nil got %v", got)
+	}
+}
+
+func TestMapMean(t *testing.T) {
+
+	tests := []struct {
+		input  []point
+		output *meanMapOutput
+	}{
+		{ // Single point
+			input: []point{
+				point{0, 1, 1.0},
+			},
+			output: &meanMapOutput{1, 1},
+		},
+		{ // Two points
+			input: []point{
+				point{0, 1, 2.0},
+				point{0, 2, 8.0},
+			},
+			output: &meanMapOutput{2, 5.0},
+		},
+	}
+
+	for _, test := range tests {
+		iter := &testIterator{
+			values: test.input,
+		}
+
+		got := MapMean(iter)
+		if got == nil {
+			t.Fatalf("MapMean(%v): output mismatch: exp %v got %v", test.input, test.output, got)
+		}
+
+		if got.(*meanMapOutput).Count != test.output.Count || got.(*meanMapOutput).Mean != test.output.Mean {
+			t.Errorf("output mismatch: exp %v got %v", test.output, got)
+		}
+
+	}
+}