Default value should not be nil (#2995)

* query/query.go: default value should not be nil

types.Marshal expects vars to have non-nil values, this change
set empty string as the default value for a referenced aggregate
value. This should prevent nil-reference panic in types.Marshal.

* query/query.go: retract default value change it breaks aggs.

Value var aggregations use nil values for an optimization, so don't
assign a value to defaults.

* types/conversion.go: add exception in Marshal for default->string

The marshaling of default value vars to string causes panic because
the incoming value is nil. This affects a narrow case so we check
that in fact it's a default value var and marshal to empty string.
In theory nothing should be nil by the time we Marshal, but it's
an edge case.

* wrong issue referenced in comments

* types/conversion.go: general case in nil value mashaling

If we are trying to mashal a nil value, return the zero value
instead of crashing on nil reference.

* types/conversion.go: replace the original to-value

* types/conversion.go: added check for Marshal to nil object

types/conversion.go

@@ -336,11 +336,22 @@ func Convert(from Val, toID TypeID) (Val, error) {
 }
 
 func Marshal(from Val, to *Val) error {
+	if to == nil {
+		return x.Errorf("Invalid conversion %s to nil", from.Tid.Name())
+	}
+
 	fromID := from.Tid
 	toID := to.Tid
 	val := from.Value
 	res := &to.Value
 
+	// This is a default value from sg.fillVars, don't convert it's empty.
+	// Fixes issue #2980.
+	if val == nil {
+		*to = ValueForType(toID)
+		return nil
+	}
+
 	switch fromID {
 	case BinaryID:
 		vc := val.([]byte)