fix NullString issue in several endpoints (#245)

Co-authored-by: Vincent Barrier <vbarrier@kagilum.com>

src/NullSupport.go

@@ -5,7 +5,9 @@ package main
 
 import (
 	"database/sql"
+	"database/sql/driver"
 	"encoding/json"
+	"errors"
 )
 
 // NullInt64 is an alias for sql.NullInt64 data type
@@ -47,15 +49,24 @@ func (nf *NullFloat64) MarshalJSON() ([]byte, error) {
 	return json.Marshal(nf.Float64)
 }
 
-// NullString is an alias for sql.NullString data type
-type NullString struct {
-	sql.NullString
+type NullString string
+
+func (s *NullString) Scan(value interface{}) error {
+	if value == nil {
+		*s = ""
+		return nil
+	}
+	strVal, ok := value.(string)
+	if !ok {
+		return errors.New("value is not a string")
+	}
+	*s = NullString(strVal)
+	return nil
 }
 
-// MarshalJSON for NullString
-func (ns *NullString) MarshalJSON() ([]byte, error) {
-	if !ns.Valid {
-		return []byte("null"), nil
+func (s NullString) Value() (driver.Value, error) {
+	if len(s) == 0 { // if nil or empty string
+		return nil, nil
 	}
-	return json.Marshal(ns.String)
+	return string(s), nil
 }